Oftentime you can go through several sets of pads on a single set of rotors. The only time you need to replace your rotors are when they are old and rotten (extremely rusty on the inside(vented) or rust spots on the rotor surface), or if you have a vibration when you step on the brake pedal(rotors are warped) OR if you hear a GRINDING noise coming from your brakes. That grinding noise means your pads are toast, you've worn all the pad friction material off and the pad backing place (solid metal) is now grinding against your rotor. In this case the rotors most likely have to be replaced, and at the very least resurfaced(only if you didn't drive very far when they were grinding, or else the rotor has already been worn down quite a lot.) ALSO if you drove for a while while they were grinding this causes excessive heat which can be damaging to the calipers, so make sure you check that the calipers retract properly when putting on new pads and rotors.