I have two cats, and I clean the cat litter every day. My cats hate to go in a dirty litter box. I also hate it when you walk into a house and it has that smell of cats.
Scooping daily will help to prolong the life of the litter. It really depends on your nose. I would say for only 1 or 2 cats, the litter needs to be dumped all together about every month. It is easier to go by what your nose tells you. When you can smell urine even when you clean the box, it's time for all new litter.
It depends on whether you use clumping litter or the gravel-type litter. Also depends on the number and size of the kittens.
If you're using the stuff that doesn't clump, I'd say once per week per box per adult cat. 2 cats with one box=twice per week. 2 cats with 2 boxes=dump both boxes once per week. Scoop poops daily.
Scoop clumping litter everyday, once per box per adult cat. Change all the litter and wash the box once per month, more if there is a smell.
I think the frequency of dumping litter depends on what you are using - clay does not last as long - BTW - always scoop everyday - clumping last a long time - those pellets let you know when they are ready - don't know much about those white crystals. Linda