I read a fair number of webcomics, and several I've followed for years now, or in some cases to their conclusion.
Judging by what I stick with, I favour updates twice a week if possible (say Mon and Thursday, or even irregularly within a week, just as long as I know I have a new page or so waiting for me Sunday), one page at a time will do me fine - but it helps if things are structured in chapters so a quick re-read of the last 20-or-so pages hangs together as a story. A page a week is the minimum I'll be bothered with - unless I really love your stuff, say like the seemingly randomly updated Subnormality! (
http://www.viruscomix.com/subnormality.html), where at least most entries take an age to read.
I don't mind a longish break if the creator needs to build up some lead-time, but I prefer it scheduled in advance ("There'll be no update until Feb 1st 'cos I'm taking a trip" is preferable to me frustratedly logging on day after day with no updates - after a few weeks of that I tend to forget to check in again, and that's that).
I dislike bells and whistles gimmickry intensely, a basic comic page or newspaper strip on my screen is what I'm after, although I've no problem with odd page sizes and layouts that computers facilitate (but bear in mind that any potential future revenues may come from selling print collections).
I like an interface that allows me to page back and forth easily without selecting dates from an archive, or worst of all, having to scroll backwards down through a blog-format.
Finally, I have absolutely no problem with something so organic as a 2 page-a-week comic drifting away from its original intentions into something else entirely, say following minor characters instead of the original lead, or shifting from gags to drama. Spread out over such a long period, and subject to constant page-by-page feedback, there's a Dickensian flexibility to the form that's part of its charm - I'm not necessarily looking for the narrative precision of Watchmen in something that is created in this manner. Not that I'd mind that either.
Not saying deviating from any of these points is a deal-breaker for me, or that they automatically guarantee my interest, but looking at the webcomics that have held my attention for a year or more, they all have most, if not all, of these features in common.