absolutely not enough 
Weeeelll, maybe not enough. On good days I'm ~30 pages ahead or more, which makes me almost 5 updates ahead. But right now I have just material for 2 updates, maybe, and I'm going crazy hahaha I spent days writing the script and not drawing so I lost all my buffer content, I fear having to go on hiatus again because of that D:
Has it ever come in handy?
Yes, a lot of times.When I'm too burnout to draw, when I'm too buzy with work/life in general, when I'm sick (I have a lot of mental disorders that make me function less etc etc), so I depended a lot on my buffer content on those 2 1/2 years I've been posting comics here. It's important if you want to keep constance and something unexpected happens you know?
At least 3 months worth of content! I update twice in a week, so that works out to be around a minimum of 24 episodes.
My buffer always comes in handy lol. 
My day job has me in either a busy or hellish busy mode, so I gotta prepare that buffer for the times when it gets hellish busy to the point that I can't spare time to write for weeks on end.
Three to four months' worth! For me that means 24 pages at the bare minimum, but it's usually over 30. I love being able to have the wiggle room to do edits or insert entire new pages on the fly ahead of time.
I started with 30 pages of buffer, and I still have 25 after removing 3 pages because they weren't really important to the story, taking a week off of drawing once for family reasons, and the natural 'buffer decay' of drawing four pages a month... but sometimes having 5 updates a month.
Either way, if I stopped drawing right now, and continued only posting 1 page a week as I have been doing (this MIGHT change temporarily for an upcoming fight scene), I could technically keep posting until February 4th before I ran out of pages.
Has having such a large buffer come in handy? Not yet, but I made it this large as a 'what if' fallback. What if I have to move? What if I get sick or injure my hand? What if I'm dealing with something really stressful for a long period of time (family illness, etc) and just CAN'T make myself draw? You know, mid-to-long term problems that would make it really hard to draw consistently. I imagine if I ever have to deal with something like that, my huge buffer will come in VERY handy.
My buffer is dry! I try to keep about 2-3 ready but I've been behind on my comic lately--Its okay though, I've still been drawing and still pushing projects forward at least. I started with 3 buffers and today (past midnight) currently marks exactly 4 months I've been doing this. I reckon I should take a couple weeks hiatus and build up my buffer before returning to a regular schedule, or else the work I do is gonna be lower quality. I want them to look good, y'know!
Well, here's the queue for Re:Apotheosis:
That's the prologue, all 30 chapters, and the epilogue. The sequel is in editing until it gets locked down sometime in early to mid-September, at which point its 14 chapters and epilogue will go into the queue as well.
I can't speak for an ongoing webcomic - I don't write those. But, for a novel, I try to have at least 2/3 of the book finished before the first chapter goes into the buffer (or, alternately, at least 4 months worth of updates). There are a few reasons for this:
If I get sick, or something happens that makes it so that I can't write for a while (and this has happened), the weekly schedule will be kept.
I won't be caught having to add something into an earlier chapter to support a plot point in a later chapter after that early chapter has gone live. The only time I have to update a chapter is to fix a typo or make a change to the description box (which does not affect the narrative). That also means that none of my subscribers will ever see a message that an earlier chapter has been updated. A novel that is being presented as a professional product should never appear to be a work in progress.
Since I own a publishing company, and I want to be able to sell print and e-book copies once the serial is completed, I need to have the text locked down at least 4-6 months before the publication date. This gives time for pre-release publicity (so, in fact, I'm right now racing to finish up the second book, Re:Apotheosis Aftermath, so that it can release the week the sequel story - which is the main story in this book - finishes its run on Tapas).
Now, my approach may not be practical for your novel project. But, it does mean that I have never had to worry about getting an installment done in time to meet the posting schedule. It's more work for me, but it's a smoother experience for everybody reading my work.
I have about 3 weeks worth of story content and some 1 panel jokes on the side - I'm pretty determined to never have a noticeable hiatus, but I think it's fine when stories do take a break I just know that if I personally take a break then I wont finish it.
I have about... 7, 8 weeks ahead on a weekly updated comic? It's important to me to keep a buffer so I'm on time every week, and I do think this gives me more opportunity to let my drawings "rest" a little. Then I can go back and try to look at things with more of a critical eye :).
It also helps me when I get swallowed by one of my other hobbies, so I never have to sweat TOO much if I fall a bit behind.
I completely finished my novel before I was comfortable posting it, so a fair buffer I would say ^.^
As of now... I'm short on buffer, unfortunately! I only have a 2 or 3 page buffer (yeah, it might seem absurd, but that's how things are) and it's not looking good for me. I update my series weekly (just did it today) and I try to do get one page ahead each week.
I made my biggest mistake halfway through the production of chapter 2. I had quit working on it because I was working on a oneshot for a manga contest and, meanwhile, I would still post pages on Tapas and Webtoon. I had 30+ pages worth of buffer, but ended up pretty much wasting them. I haven't been able to recover since then. So yeah, once I finish posting chapter 7 (it'll be next week), I'll most likely go on hiatus and start things anew!
I remember telling Andie, my artist, that we needed to do 10 before posting. It got to a point where he was like "Can we post nooooooooooooow?". I'm glad we did 10 when we started because WHOOF. Now we do 5 before hiatuses. I plan on doing quick gags once the Season is over.
