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When I get tired of it. I usually keep my avatar for about 2 weeks or so before going blank. After that I either let it stay that way or I quickly find another image to use as an avatar.
I sometimes change it. Usually several months pass before I do so though. Most of the time I just wait until I find something worth putting as a theme, and that's when I change it (because if I were to change it, I would also change my signature at the same time).
Well, i just changed mine this morning, for the first time in 2 months, and i put it back to the very first one i used on the forums :P So, i'd say whenever i feel like it, or when i was getting tired of my old one D: