Generally I'm going to say somewhere between 2-3 months. If you keep an avatar for less than a month or so, no one's going to recognize you unless you're a very well established member. If you keep an avatar for many months at a time or forever, your avatar is highly associated with you and likely to become iconic, but there's no variety. That's why I think 2-3 months is good - it's long enough for people to become familiar with the avatar, but allows you to keep things moving.
