Very simply, when I no longer enjoy reading it.
I've ditched very few webcomics - off the top of my head:
Menage a trois: It was fun, but I just stopped reading it as it didn't seem to be going anywhere.
Unicorn Jelly: The first webcomic I read, and I stopped because the comic stopped making sense (and I was beginning to get the feeling that nobody had a higer opinion of their work than the artist themselves).
On the other hand there's several that I come back to and catch up on - Freefall and Sandra and Woo, for example - as they are best read in blocks rather than page at a time.