Much like @Iron, just the general idea but detailed scenarios come to me as I go. Script changes as I draw the pages, too. I'm never one for intense planning. My way of doing it just keeps me more interested in the project and avoid burnout.
Yeah like Iron I just have a vague idea/summary of what a chapter is about (in relation to the plot of the whole story) and call upon my patron god Bullshitticus to guide me through the rest of the way.
When I start writing a new chapter, every chapter after it is a bigass question mark. I make sure I define a beginning and an end for each chapter though and work on the innards once I get to it.
@elixiadragmire WORD.... drawing off of unfinished script sure is exciting
For Demon House I had like 52 pages of script all ready in a Word doc and clear character designs before actually starting to draw and post it. But I don't work with a buffer so I can freely change up the dialogue and interactions of a scene as I go, especially when I get a feel for what the audience wants. And this did help for next week's episode.
For my newest series, Heavy Horns, I had a much less developed idea. I had the character designs down and the main theme of the story, but I posted a chapter image before even thinking of a set storyline. : p
But ever since I posted the chapter image, I had been getting a lot of subs so I've been pressured to write more for it. And I have! I've now got a viable story for it developed in a few days. I just hope I don't disappoint (•- •;)
As for how long it takes to complete pages, for a two-page episode of Demon House, I'd say around 7 hours, Heavy Horns takes less than that because it's in black and white.
I've written down in a little notebook the basic vague idea for at least the first 12 chapters of my two comics Jonny Comic and Valve Comic (I'm up to ch 3 for Jonny and ch 1 for Valve). Each chapter introduces a new villain, so that that's what breaks up the stories. In between that I have character arcs and continuing questions for both readers and characters. I'm pretty obsessed when it comes to story plan, so I make sure to have my comic's saga all planned out for the most part.
Then I do a chapter specific plot graph (rising/falling action) and put down what each page will show, whether it's an action scene or quiet character interactions. The problem is that, with the how my uploading schedule is, I do a minimum of 9 pages a month for both series'. And since each chapter is 24-28 pages long, there's at least 2-3 months from when I start and when I finish. In this time new ideas spring to mind and things I thought where important can get dropped. For example near the end of drawing Jonny Comic ch2, I actually dropped three pages in my graph because they where no longer necassary.
So even though I throughly plan out each chapter before I start there's still a little bit of winging it involved.
Constructing the story, for me, is equally finished and incomplete. I spend time to make a clear structure to the story so I know what happens in what order but some of the little details I wait until Im starting that chapter to tackle. Since it will be years before my comic will be completed, I want the freedom to have wiggle room to improve it naturally as I grow too. Besides, new research and experiences in life provide things that may have been missing from the story to make it better. For example, I am writing a story involving corporate shady business and I had an idea kinda how I wanted them to wrong the main characters, but I accidentally stumbled onto the very thing I needed for the story to be believable for its time period: COLIs1 So to answer the question, only partially written before starting comic production.
The story I'm developing at the moment actually started out with a rough scribble on my phone. Somehow I started to imagine who the characters might be, and how they ended up together, and slowly some kind of plot was born, more characters were added and the plot rewritten and fleshed out.
5 months later I'm sitting here with raw events for about 15 chapters (still missing major plot events), rough storyboards and dialogue for the first ones, 10 more or less detailed character bios that are constantly edited and extended, and I haven't drawn a single thing. I'm constantly questioning my characters' motivations and decisions while further developing them and the story. This leads to a lot of retracing actions and events to make sure everything fits together and creates a believable environment with authentic characters. Perfectionism is probably going to kill me before I produce anything...
@patrickj Developing and building and laying the groundwork for a story is very important, but there comes a point where, having settled on the major plot-events, you kinda have to stop second-guessing yourself and start doing instead. It doesn't have to be perfect - it just has to be done.
Like Neil Gaiman says, the most important thing you can do is finish things.
Second this!
I can't tell you how many 'writers' I have spoken to who have drawers or drives full of ideas, chapters, and other partially finished, or even finished stories, but have never done anything with them. Submitting or self-publishing is what makes you creator, don't keep that stuff bottled up where no one can see it. The only way to grow is with critique and exposure.
Eagle
(And it's hard to put it out there, but the rewards make it all better)
That is the one reason why I actually created my comic. I had the story developed some what but I didn't think it was good enough. But the I learned, that I shouldn't get overly attached to my story trying to make it "perfect" and finish it. Then, after that use what you learn for next story.
This all returns to my days on Flipnote Hatena where I started my main webcomic Two Faced.
Before that I had drew the comics in school books... that I stole, ahem yeah
well I worked from those, after I ran out of books I just made it up as I went along...
(Does not recommend) ^
NOW I'm left with 107 episodes and a series full of plot holes. It took a while just to sort out the start of it, and was a total mess.
Well I just sat down and figured it all out. Now I have a clear end to the story...
It took me two tears to develop the story to Shadosassins. I didn't like the story I started off with and it took me quite a few re-writes to get the story to where I wanted it and that included its environment. I left small holes in my story that I can change on the fly if need be just so things didn't seemed overly structured when I actually started making pages. I my story has a defined beginning, middle, and end, but I left little areas of how I get to these points changable.
4 months later
bout 40 years in the making 
their are elements of Tales From Swipe City that are reflections from my life. one of the characters in it is based on my mother (nobody tell her that !) another character is based on a very horrid teacher I had. and one or two other things that happen as the story moves on are based on events in my life ( just hyped up a little) .
one tip I will give people is i sometimes plot events backwards. that way you can move backwards along the time line until you work out the best place to link it to a current running plot line.
Starting from the very first scribbles of characters in the notes of one of my college classes to "get the idea out of my system" so I could work on Serious Projects (hahaha) to me posting the first page online was somewhere around 4 and half -- 5 years. At first that was just me pondering ideas, sketching character thoughts, considering the world. Then, when I finally realised this wasn't some dumb idea I was gonna drop after a month, I started trying to make a rough outline of the plot, figuring out how different events would lead to each other.
Runewriters started out with 3 main characters and a couple of side character concepts. I realised what the story was about after I finally conceptualised one of the major villains. It now has 3 main characters, 3 other major characters, 3-5 main villains depending on how you count, and a couple of side characters. I, uh, have a tendency towards ensemble casts,,,
When I first started posting Runewriters online, I had a half-finished script of Chapter One, a rough outline of events for Chapter Two, and things like "this is where they meet character A" "this is the one where so-and-so fights such-and-such" for the rest of the chapters. Somewhere in the middle of drawing Chapter Two, I completely rewrote the order of events for Chapters 6-9.
Currently I've just finished Chapter Three, I have a full (rough draft) script for Chapter Four, I'm working on a more detailed outline of events for Chapter Five, and I have rough "here are the things that need to happen in this chapter" notes for everything else that I think about constantly in my free time. I like thinking ahead but not laying everything in stone just yet, to give myself some room to have new ideas!
And it takes me about 10-12 hours to finish a page because I'm a huge slowbie. 
;;
world at strife started out as a little idea my friend and i had back when we played this social media game called "TinierMe". it was basically a shittier version of gaia online but we had fun with it all the same. i met this friend through TinierMe and we joined a group to discuss games and stuff. at that point i was in middle school drawing and i figured "hey, i can make a comic about our group talks and stuff" and i started doodling cover pages and junk. as I grew up i started to think my old work looked like crap and once TinierMe started to dissolve my friend suggested i make a comic about our OCs from TinierMe, which would later become the main characters in World at Strife.
world at strife was an idea we pitched ideas for for years. when i came back home from school i would talk to my friend over skype and we'd come up with goofy plots and characters to add to the story. after a while i actually tried making other comics as prototypes to see if i could do it. all of them failed, and i stopped working on comics for about a year. in 2014 i finally got the resolve to do WaS, which would eventually be tempered into what I have today.
i planned out the beginning and the end, i just wing the middle.
19 days later
I have a tendency to come up with huge amounts of different characters, combining some of them in one story idea and trying stuff out by just sketching character interactions and such and working from the perspective of the characters and their chemistry. Some work, some don't and the ones I've grown most fond of tend to stay with me.
I've really been into comics for as long as I can remember so working on the story by simply writing it down isn't really natural for me so I mostly come up with really basic plot points and only write dialogue for character interactions down, the environment, body language and such feels really difficult to describe in words but come out more naturally in drawings. I've tried to get to the habit of actually story boarding my pages beforehand these days because that way I can actually show them to people for critique to make sure things come out as I want them to and the flow doesn't confuse the reader.
I used to brainstorm stories a lot with an old friend of mine but since ours interests story-wise have floated in different directions I'm mostly just work out the stories by myself maybe getting a second opinion here and there from someone else..
It took us a single day to come up with Tales from the Well XD we had to pitch the story for class the very next day with plot, character designs, and a summary of the first episode.
I agree with most of what was mentioned here for plot development. Rough plot idea with major story points planned out. Enough to have a plan and enough to be flexible =) also keeps you interested in the long run!
Hmm... that's actually pretty difficult question, since I re-vamped my comics there were 2 ways I did the plot thingy. First one I was bored and... and needed a man (XD) So I started to write short story. It was basically a soft erotic. I didn't thought much on the plot, I would make it a one-shot. Later my friend was like,: develop it more! And I was like: OKAY :v and I started to write more. I was just going with the flow, but to be honest, it created a huge plot holes when I tried to add more and more things, so I would recommend not starting your story not knowing the ending.
When I re-vamped my story for 2 months I was writing the script, AFTER I knew where the story is going, so my process looks like that: IDEA ON MY HEAD > WRITING STUFF ON TINY SHEETS > START TO WRITE THE WHOLE.
Before I started to draw (since I don't really do storyboards) I had 8 chapters done (about 15 pages). The script isn't finished, but I have already planned the main events in the story, so the smaller ones to connect the dots are less difficult. And I'm sticking with it, with only small changes in the dialogue.
How long does it take me to finish one episode? Let's see... 30 or 40 pages... it will be about a month or a bit longer. Sometimes I re-draw pages, that didn't turn out how I wanted.
Okaythatsallbyeeeeee.