I very very rarely unwatch people.
I will always unwatch someone if they aren't the person I thought they were, usually. Like if they seem like a nice person at first, but they actually are far from it, regardless of how much I like their art.
I'll also unwatch if they've been inactive for a while.
very rarely I'll unwatch someone I watched a long time ago, and they haven't really improved much/at all, and I've lost interest. That happens very rarely though