How much material is "typically" taken down on a rotor
when turning them ? Outside of unusual run-out or grooving.

My machinist only removes as much from each side as required to get a completely clean groove-free surface. So it depends on how deep the grooving or warps are on the rotor being turned and how deep the grooves are. there's no real set amount; only a minimum thickness for the disk post turned. . .

with the above but...

For cars used in perfomance driving situations, I never turn the front rotors. In theory (actually, fact) the rotors absorb heat in order to counter the energy of motion. The less mass the rotor has, the less heat can be absorbed and it follows that the brakes will fade sooner. For street driving, turning your rotors is a perfectly safe and acceptable practice.