I don’t know about you but I change my desktop about once a week. If I find a really good wallpaper then I might keep it for a month. Not that I have a rule about changing my wallpaper, I just like to.
To put my Cinema Display in the Christmas Spirit I’ve had 1920×1200 pixels of Kevin Cornell’s Christmas-style Desktops up all month. And speaking of which, he just put up a new one, and wow. Kevin is talented.