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There are wear limits. These are:
New:
Front: 24mm (0.94 in.)
Rear: 20mm (0.79 in.)
Wear Limit:
Front: 22.6mm (0.89 in.)
Rear: 18.6mm (0.73 in.)
Machining Limit: ('after' machining)
Front: 22.0mm (0.87 in.)
Rear: 18.0mm (0.71 in.)
Plus, there's a Peak-to-valley surface roughness measure of the brake disc after machining, which is: Max. 0.006 mm.
Finally, the Lateral Runout of the brake disc, which is: Max. 0.03 mm.
You need to use a mic or caliper on several spots around the rotor and avg. your readings to determine replacement or not, and a dial gauge to measure the lateral runout.
The rule-of-thumb is that you get two sets of pads out of each set of rotors (street pads - if more agressive street/track pads are used, you need to adjust for this). Again, pads usually last 30k-35k mi. You can use the car's mileage to roughly determine if rotors are needed. I would say that if you have 55k mi.+, you should probably be looking at rotors too. Again, you need to take into account the driving style too, these 'rules' are for 'normal' use. Total freeway miles and you can extend this a bit. Total in-town stop & go, or track, ax, or de days and you should use reduced limits.
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