I think its really down to how used they are, sometimes you can get aways with shaking off or out poops and washing them once a week. If they wee on them a lot i would wash them as needed.
My older boys are horrible, they live in their cozies and they get wet, poopy and food covered so quickly, so mine get washed every other day and replaced with a new one. They go through them so quickly i had to make about 12 of the things.