I have two extra giant litter boxes for 4 cats and I also keep about 2-3 inches of litter in them. I scoop once a day and I use Ever Clean Unscented Extra Strength Formula and sometimes the scented kind too. It's quite expensive at Petco but I've found it to be only 10.99 normally or 8.99 on sale at Pet Club for a 25lb box. I never change all of the litter completely unless I change to a different brand or unless it smells very bad. I also use litter box liners so the urine and poop never touches the litter boxes so odor doesn't become embedded into the plastic. The litter also lasts a long time. I add a little bit to the litter boxes about every 2 weeks or so.

I've tried many kinds of litter and have found that Ever Clean is much less dusty, clumps very hard, and controls odor very well. My cats also seem to like it and usually cover up all of their poop now.