If you have a yard that is too rocky to plant grass in to where it will actually take hold and grow, how much topsoil is recommended to where the grass seeds can actually take hold and grow? The area in question is roughly a quarter acre.
If you commit to lots of water and more than average feeding, you could probably grow turf in 6-8" with 12" being better of nicely ammended topsoil. You are going to have very little draught tolerance since you will, in essence, be growing a sod layer over rock for eternity.
You would be better off seeing if there are ground covers or native plants that would be better adapted to what you have. Or at least cut the amount of turf you have in mind from the 1/4 acre you need to adapt.