Eight weeks or when mom weans them. You should never force a weaning. Just like with humans, a kitten (or puppy) will get antibodies from a healthy, vaccinated momma, and the longer they are allowed to nurse, the more they will receive. Momma will be more than happy to let you know when it's time for them to leave the nest.
And like others have expressed, most momma's will also litter pan train the kittens if you give her enough time.
Anne