It's not really about time, there just isn't the functionality. And to code something like that would just kill the load on the server. It adds load just with the 75 or so styles we have, but imagine with the ten thousand or whatever avatars we have. For the software to compute that every time someone looks at the avatars...
Frankly, it's much better for everyone if you just order the member list by last online, and look at the avatars people that are here are using. Or pick one you like and if you see someone else with it, change. Because the numbers would be misleading anyone - if someone who hasn't been here for 6 years is using the avatar you have, what do you care?