I normally keep my background the same for very long periods, simply because I almost always have something up, so I don't even look at it that often. I might not be that big a fan of wrestling, or that big a fan of CM Punk, but I saw this image and just instantly thought "Wow, that'd look great as the background for my desktop..."
Until then it was a picture of the Assassin's Creed Brotherhood box art. Had it until a little over a year ago. I just change it whenever I get bored of the current image and simultaneously know what image I'm going to replace it with, which isn't that often considering how little I actually see it.