The rule of thumb is a rotor can be 'turned' once. I usually look at the lip edge of the rotor (where the pads do not touch) and if it looks like more than 3 mm higher than the rest of the rotor surface, then I replace the rotors.

While rotors need to be replaced in pairs, the front and rear wear at different rates.

Pads and rotors are easily replaced, all you need is a good wrench or 3/8 socket, some silicone brake grease, a 6mm allen wrench for the slider pins, and a coat hanger to hold the caliper up out of the way.

The rear rotors are different, as they also contain the parking brake shoes. I suggest replacing the shoes (less than $20) while the rotor is off. No need to wait for the shoe pads to delaminate!

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Keeping it running is better than buying new