I have two cats and am using Wal-Mart's "Special Kitty" clumping crystal litter for multiple cats. I scoop the lumps twice daily. With this litter there is no odor, it clumps really hard and seems to be the best I've used so far. My question is, how often should I actually empty the litter and wash the litter box. As I scoop I add new litter each time so there is no build up of old urine & poop. Empting the box is a chore I don't like. Would appreciate advice.