It typically takes between 5-10 days for a mother cat's milk to dry up.
The process of a mother cat's milk drying up is often referred to as weaning and it can be an uncomfortable experience for both the momma cat and her kittens. During this time, momma cats may become increasingly irritable or hostile towards their babies as their bodies start producing less and less milk. In the meantime, it is important that the kittens are provided with a good source of nutrition in order to ensure they stay healthy during this period of transition. If you suspect your cats are going through weaning, make sure that you provide them with plenty of food (including wet food if possible) so that they can get all the necessary nutrients while also helping momma cat’s body gradually adjust back to its pre-pregnancy state.