How often do you change your phone's wallpaper? (poll results)
You can often tell a phone line's generation simply by the stock wallpaper used
That being said, though, there are plenty of approaches to wallpaper-ing. Some just stick with whatever their phones ship with, as long as their icons are easily discernible on it, that is why the stock wallpaper collections are perhaps the most widely used ones, and some have become a signature of whole phone lines or generations.
Others would change their stock background with something else that is provided by the manufacturer out of the box every now and then, while some would go to greater lengths and install a wallpaper app that gives them access to millions of tailored images to pick and choose from. There are even apps that will change the wallpaper for you every day if you wish, so that you never get bored looking at one and the same screen. You have to be careful with automatic swaps, though - you don't want to end up with a bright set of icons on a bright background, or vice versa.
Things that are NOT allowed: