How far do you have to drill to make it to the middle of the earth?

You could not even if you wanted to. You would not be able to transfer enough torque to the drill bit. The pipe would twist in two and shear off. Even if you could get the torque, once you got through the crust, your drill bit would melt in the magma. Also, not drilling mud would be dense enough to keep the magma from flowing to the surface.