-When it feels like all the setup isn't going to pay off
-When I realize the comic is less of an actual story and more of a slideshow for fanservice
-When I stop liking the art
^That last one is a big one; I've dropped many comics over that.
It's not that I'm shallow; it's just that my expectations for writing in comics are already fairly low. I'm used to following along with a story that appears stupid/boring as long as the art holds up. Comics need time to get rolling, after all; it's possible that I just have to wait to get to the "good parts".
But when the art doesn't do it for me anymore...what's the point in sticking around at all? I'm not going to fully enjoy a story I don't even want to look at, so I might as well just leave.