How far beneath the earths surface would you have to go to be where there is no more granite?
Below the crust. This varies based on the thickness of the crust
and it has to be continental crust for there to be granite. There
are some exceptions to this but it is very complex and not very
well understood at this time.
The crust varies in thickness between ~25km and 70km (Basin and
Range province and Himalayan Mountains, respectively).