Brake rotors have a pretty long lifespan compared to the brake pads themselves.
Most times you'll go through a few sets of brake pads before you need new brake rotors.
The only real way to know whether or not your brake rotors are bad and worn out and need replaced is to remove your wheels from your vehicle and actually look at the brake rotors.
If you notice the brake rotors are warped then they need replaced.
Or if you notice cracking on the brake rotors surface or the brake rotors are wearing out as in getting thinner and thinner then it's time to replace them.
Most times brake rotors will last 100,000 miles to 200,000 miles depending on your driving habits.
If you drive more on the highway then your brake rotors and brake pads will wear out less often but if you do a lot of stop and go city driving then your brake rotors and brake pads may wear out much sooner.
Some brake shops will do a free brake inspection and can tell you whether or not you need new brake pads, brake shoes or brake rotors or brake drums.
If you hear any metal to metal sounds when braking or hear screeching when stopping the vehicle then you either need new brake pads or brake rotors.