maybe about every month? it usually depends on what collectibles i just bought or feel like displaying, and everything tends to follow that based on the collectible colour scheme. however, sometimes it doesn't really coordinate, like right now aha
only when required to. one of the advent events had everyone changing their avatars to include a santa hat. that was probably the only time i changed my av.
my signature won't change unless my town info changes...
I usually change them seasonally, like I'll have a Halloween theme, and a Christmas theme and stuff like that, but I am really content with my theme for now. Once it gets closer to Christmas I'll probably have my Christmas lights and collectibles out and have some Santa hatted avi. =D
I've only had two different avatars (not counting holiday variations) during my 4+ years here, so not so often. I change signatures, collectibles and user title a bit more often though.
I tend to change it when there is a make your avatar and sig contest. I do want to make myself a Fire Emblem sig, but I have been too lazy to, so for now it is still the retro one from the fair.