I believe that the "one year"-part has been disabled now, so you need 500 posts. I'm not entirely sure why it was disabled, but I think it got purged when they updated the boards to the latest version of vbb.
It might be that they've activated it again, for all I know, and in that case, it should be whatever comes first, and you should be able to do it.
I made my custom avatar when I was still able to do so due to my long, faithful membership. I only recently hit the 500-mark.