Usually around 4 - 5 weeks. Mom will show them where the food is when she is ready to start weaning them. Litter boxes are needed as soon as they start eating wet food. Use Pate type wet food at first, mixed with KMR or water so it is like a mush and easier to eat.
Use Non-clumping unscented litter. The kittens usually try to eat it at first and clumping litter and clump and block their intestines.