I keep mine for three, four, maybe five months and then change it unless I find something that looks a lot better in which case I'll change right away.
Not especially, but people do come to associate you with your avatar, e.g. Majed's posts always catch my eye, because I know that avatar like I know a friend's face.
Can someone do something with this for me please? I'm not sure what colour the border should be so feel free to experiment with it. Just the face and the top of the shirt like my current avatar.