If the brake pads ore worn down to a couple millimeters or less, it's time. When the wheels and brake calipers are off. you can measure the thickness of the rotors to see if they can be machined and still be more than the minimum legal thickness. You need at least .020" over and above the minimum thickness in order to machine them. Often, I prefer to replace the rotors if the price of new ones is anywhere near the price of machining. I'd say, replacing the rotors and pads would take about 1 to 1.5 hours labor time. Machining woud add to that about 0.5 hr per wheel.
generally, rear drums and rear brakes last a longtime, 50,000 miles, because the front rotors are what is stopping the car. Toyota corolla's front rotors are easy to replace, a real do it yourself thing, and I suspect that most American cars are as easy.