Litter Box cleaning should be done daily to remove any cat feces from the litter box.
As to how often you should replace the cat litter that should be done at least twice per week.
So daily cleaning of the feces and then once or twice per week you should change the cat litter and wash out the litter box.
If your cats litter box isn't kept clean enough then the cat may decide to start peeing or pooping on the floor instead of in the litter box.