Cats are very clean animals so they prefer clean litter boxes. Every day you should scoop out the waste twice, or at least once, and replace the discarded litter with a fresh one. Every week empty all the contents of the litter box and scrub and wash the litter box with warm water and mild soap. If using clumping litter you can wash the litter box every month. Every year, or every six months if you can afford to replace the litter box.