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.