Not often.
I will unfollow a question if I happen to be spending a lot of time on Fluther and there are frequent new answers of no substance (like if two people start having an unrelated discussion which happens not to interest me).
I’ll also unfollow a question if it’s a simple leave-your-answer-here question, and the answers turn out to be less interesting than I anticipated. I can’t think of a good example of this off the top of my head. I guess the party questions are kind of like that. I’d rather drop in from time to time than receive alerts every ten minutes.
I don’t ask many questions, but once or twice I’ve unfollowed my own question because the answers were spiralling off in the wrong direction after I’d already received good ones.
<shrug> Fluther’s not all that busy lately. If I unfollowed every question I left an answer on, there wouldn’t be much to see here. But I guess, looking over my own answer, what usually triggers me to unfollow is if I don’t want a specific question to trigger the “Activity for you” count. I like to keep the count at zero, so I always visit the questions that pop up there. Some questions I just don’t want to visit frequently.