I would stop watching them if I didn't like their work anymore for some reason. Also if their personality starts to show itself to being something I don't like, like if they start getting really rude or something.
They're style change is my main reason for ever unwatching people, but sometimes my inbox becomes too crowded and I will unwatch people who upload too much. And once I unwatched someone because they were very rude and unpleasant to their watchers. (I think that's a rare situation, though.) I really liked their art, but not enough to deal with their attitude. ):
there was this kid [acctually a year older than me], who watched me like a year ago he hardly ever commented on my work but whenever he did it was always negative, or critical, i can take criticizm but as long as its constructive, he commented recently after months and as usual it was another negative, im not gonna have someone watch me and say me works crap all the time so i removed him
These are not in a specific order, but I'd say I stop watching people if:
1. they turn out to be arrogant and way into their own work to realize that there's always room for learning and improvement 2. they're young when I added them and during the course of time their work doesn't seem to show any sense of being improving whatsoever 3. their work turns out to be repetitive and dull in the medium/long run and I start getting this feeling of "been there, seen this, it's time to let go" often
There was one guy that started talking smack about a very well known web-artist. I get what he meant, but it was a little too rude and happened way too frequently. When it became too irritating, I stopped watching.