I haven't cleaned my computer since I got it which was almost three years ago. I need to go through and delete videos, images, and programs I no longer use. I also need to physically clean out my computer tower too with my electronic duster.
I usually go through a couple times a year to delete things I don't need or use anymore. I try to keep things pretty organized on there so I don't have too much clutter to begin with, which helps a little.