i would say that you should just update it as often as the friends list does? from my understanding, doesn't that information get pulled along with the friends list? or do you do it some other way where the current user's header doesn't show up with it?.
if not with the list, then i would say the more often the better, people arn't going to be looking at it unless there was a change, and the only way to catch when there was a change would be to update it as often as possible, im sure it wouldn't effect the bandwidth usage any huge amount?