I never really had a time limit only if they got bent, scored or worn out of service. if you look closely at the rotor it should have a minimum thickness stamped on it , if not refer to your manual. You maybe able to resurface them if they are not in bad shape. In fact I don't think I ever changed a rotor because it wore out, since I use softer /cheap pads. I would rather change the pads often than the rotor. Going to metallic or ceramic pads will last longer at rotor life expense