Depends... I usually change it about once a month. But I also change it if I find a cool, avie sized picture. Like the one I have, the Bad Girl one, I found on a site that has a bunch of cute little pictures like it. Sometimes I'll also find a good Teen Titans picture and crop out some of it to be avie sized, and use that for my avatar.
And all this talk about avatars makes me wanna get a new one. XD