Monthly-ish.

weekly. sometimes daily.

depends. I had a wallpaper changer, pulling a new wallpaper out of the sack on every (re)start. But now, I mostly change it manually, average life time some days

I change it whenever I see a particularly nice one. Both my computers currently have hameed wallpapers.

less often then my underwear.

eh depends. At times i'll change it three times in a row. and then there are some that i wont change for days. The maximum ive ever kept a wallpaper.....5-8 days running. Minimum....a few seconds. I changed it immediately

Well, whenever I got fed-up with the current one...

Depends on the wallpaper, if it's a niteangel wallpaper, I'll have it for months, if not maybe weekly.

every 5 minutes
gotta love OS X

Only when I come across a new one that I want.

I rarely see my desktop anyway.