With TWO CAT'S using the same box, litter should be replaced with a 7 to 10-pound bag every 15 days. That's easy to remember. And it costs less than four dollars a month. Even clumping litter has a saturation point. Usually caused from small partials that fall away from urine clumps and back into the clean litter.
Guidelines are tough to call for complete replacement of clumping litter.
We found trusting our own nose really worked best in deciding when our clumping litter had reached it's saturation point and needed to be completely replaced. A hand held scooper is an important tool for any litter box. For whatever reason, some cats don't always bury their fecal droppings. Yes, at least once a month - or more often if you prefer. Hot water is all we use.