The ones I've used here have only been 4 over the course of all the years I've been here. Which is a surprisingly small amount since I probably would have had a new one every month in previous places. Or maybe I've just found avatars for myself to be less important to change all that much. But whatever, aside from the skull I have now (it's from a photo I've taken), I've only used these three;
As to why I change, probably for the same reason I decide to cut my hair. It's one of those things that you see and go; "Man, fuck this haircut." But until that point I don't actually give it too much thought.