There's a graph where X-axis is labeled "frequency of updates" and the Y-axis is labeled "quality of updates", and there is a steep cliff dropoff. If you're below the line, you're out.

I'm willing to keep regular with comics that update frequently (and ideally, regularly), even if the quality is maybe a bit meh. XKCD, QC, and GrrlPower update like clockwork multiple times a week, so I stick with them even when they get meh. S*P doesn't update as frequently or as regularly, but the updates tend to be more enjoyable so that's fine. I jive with it. Girl Genius is great on both axes.

Most comics lose me because they just stop existing. Or at least stop updating. Once in a blue moon this will be because the creator finished the story they were wanting to tell, but it's an easy bet that when a webcomic stops updating it's because it's died and the fans just don't realize it yet.

Then there's comics that you realize died somewhere along the way and are just still shambling around in their corpse. They were once funny, or nice slice-of-life, or hinted at plots with a lot of promise. Sinfest is the primary example, since it's zombified twice. But also if you're anything like me, your webcomic graveyard is probably full of a whole bunch of comics about teen-ish boys and some manic pixie dream gal who's just a friend but always on the verge of becoming more than friends, and at some point you realized that none of these comics would dare to advance the relationship because the "implying they might hook up next time" schtick is how they keep you coming back one disappointing update after another. Comics that had potential to explore some interesting, sometimes even unhealthy, relationship dynamics, but never had the balls to go further than having a crush on a pretty girl, and you realized they were never going to explore that potential - that refusing to explore it was the hook.