It varies based on the rotor's manufacturer. It will be stamped on the "hat" of the rotor, usually on the outside. Generally, you need to do a rotor change every time you do a pad change, that is the way they were designed. The only exception would be if you use ceramic pads, which are much easier on rotors (which is part of the reason they create less dust). Ceramics can go rotors every other pad change, depending on how you drive.