Hmmmm, I don't think there's really a minimum for slice-of-life webcomics. They're more flexible and are constantly evolving, so maybe the artist becomes better at writing characters by the third act, who knows? 
I guess it doesn't really matter. With this format of webcomic you might have a larger cast and the reader can pick and choose which ones are their favorites.
One very appropriate gif... applause
Two, I only ask because I've been on a bit of a keeping things as short as possible to allow me to explore a genre to a point and then move on to something new so I just want to know what I can before going into the SoL genre for something that I already wrote the end for
I really like this question since I've been wondering along the same lines myself - specifically, how to make a story impactful while also being very short.
So far I think there is really no "absolute minimum", and the length of time it takes before a connection / empathy / caring for a character can be established is only limited by storytelling ability. Even a standalone work of art (arguably the equivalent of a single panel) can evoke some kind of connection.
I do think the potential for such sentiment is higher in longer series, since there is simply more time to get to know the character(s) in different ways. But even 20 pages, 10 pages, or a single yonkoma can do it to some degree. The only way to know the limits is to test them!
Sometimes I care about the character who have appeared just in 3-5 panels so far, if they have some catching and lovely features 
This kinda got me thinking and maybe it’s just me but I almost always go into any piece of media already assuming that I’m going to like the main character because that’s usually a given. That’s why it’s such a twist when the character is an unreliable narrarator or just a generally unlikeable lead.
I think with comics people tend to do it waaaay more slowly than I prefer though. I stop reading most comics in the first few chapters if I don’t have any sense of the main character.
That seems like a very long time to connect with a character. Do you mean 15 pages in?
I don't know about number of pages per say, because using the webcomic medium is so diverse and how people tell stories varies so much. For example, gag-a-day clips like Sarah's Scribbles, I can relate almost instantly. A single gag has drawn me in.
For more long form comics, some comics are faster to read than others (Like when they are vertical scroll like on Webtoons), then its faster to get into it and relate depending on the pacing of the story. For example, Let's Play, I cared about the characters by the end of the first episode.
Some comics it just takes for ever for you to get invested in the characters for some reason. I usually give a comic about 10 mins of reading time to see if I'm into it or not. If I have to read for longer than that and I still don't connect, I mostly give up and move onto something else.
It depends on how you execute presenting the character's personality and it depends on the reader as well. Some readers are just quicker to relate to a character and begin to like/care for them.
It could be as easy as having one single panel with a dead pigeon on the ground and a guy looking at it and saying "Mood." I know a bunch of people who'd get a kick out of that and identify with the character immediately.
Maybe it would take another panel of him petting a duckling that will have people really liking him.
Then another of him visiting his dying grandma in the hospital for people to sympathize and care about him.
So I guess the minimum really is 1 panel. The first panel your character appears in. 
I see, yeah going off of your chapter definition that's roughly the length of a comic book or chapter of a graphic novel. In that amount of time if the writing is decent I should care at least a little about the MC. Typically sooner than that though. Although I will say sometimes characters take longer for me to connect with, but as long as something holds my interest in the first several updates (maybe plot concept, style, etc.) then I can give it a little longer. But if the plot is boring and the characters are boring, no go xD
It really just depends on wheather i can identify with a chara or now and how well they are written.
sometimes I can call them my child after their first line of dialogue and sometimes I grow attatched to them only after several arcs and development
don't think there's a rule behind it tho
I don't think it's that you need a certain lenght for the emotions to be felt, rather the quality of the emotions in each panel. You can make a scene very short but very heavy on the feels or 40 consecutive pages of nothingness. It depends on the skill of the author. Don't feel obligated to meet a quota... just let it flow and you're gonna get to that sweet spot!
well the quickest I've done it in is 3 panels. People were telling me that they had major feelings for Lampchan by the end of the first ep:
Here if you're curious- nearly all the comments are about how many feels they now had XD
I mainly read very subtle/read-between-the-lines type of works so, the first panels have to be impactful. From my observation, it's the matter of panel representation, since my fav artists handle panels execution pretty remarkably to set the tone of the story right away, kind of in a cinematic sense, even when the characters aren't in it. That helps a lot for me to know how the character feels and their emotions right away, and i cheered for them quickly. So maybe it depends on how well the author handles the execution i guess, with one panel as a minimum.
In my opinion is not about a number, is about the way the character is presented. I have watched entire moves and series and never get attached to the main character because it was so gray uninteresting and generic.
I can care for a character who has been there for two pages because it is instantly engaging. In my comic I introduced the runaway kind and he literally was there in one panel and some people commented "can we keep him?? I love him !". Of course, real love grows over time, maybe get at least one significant moment in the story.
if you do it right you can make people LOVE a character with only one pannel.
Sometimes a sketch can get people interested if the design stand out.
Of course, the personality is a key element too. Sometimes just getting the character to talk (if he talks) or make a gesture can work wonders.
Sometimes the way people receive some characters can be a surprise.
Fun fact: one of the most popular villains in my comic has appeared in only 2 pannels for now.
So, there is no minimum for sentiment to set in.
The key is to make your characters feel "real", and for that a solid and consistent characterization is key. Even if they change in some ways like having some character development, is important the transition makes sense for them.
They don`t need to be realistic, but they must be beleivable inside your story.
