Traditionally, I’ve never had the same wallpaper on any computer for more than a half hour at a time.  I like my wallpapers to rotate, since I’m surrounded by 3-4 computers at any given time and usually only working on 1 or 2 of them.  This way I can have nice pictures flying by to keep my perhipheral vision entertained and people walking past my window can marvel at the 3D art I have displayed.