Originally Posted by Turb0Surge
You'd feel it if your rotors are warped because your whole car would be shaking when you applied the brakes. Your rotor won't get "thinner" or have any kind of damage under usual wear. I think your indy shop was just looking to make some more $ off of you.
The main reason people change rotors are because they wait too long to change the brake pads and they grind the rotors up. Or because they wash the car when the rotors are super hot and warp the rotors.
BMW rotors use a very low carbon cast grey iron and are NOTORIOUSLY soft, and typically will wear below minimum thickness before the 2nd set of pads.
As to the OP's initial question about how long these rotors should last, there's no good answer, to be honest. If you drive 55mph, never go over 2,500 RPM, brake like you're afraid the brake pedal is going to bite you back...Let's just say I've seen factory rotors that have lasted over 100,000 miles before. Then again, if you're living life one traffic light at a time, DRAGS your left foot on the brake while you accelerate with the right? Those rotors will likely be done by 30,000 miles. Or if you go to about a dozen local high performance driving events at very brake demanding tracks like Auto Club Speedway, where you're slowing down from 125mph to 40mph repeatedly every 2 minutes, 10-12 times in a row? Don't expect it to last more than 20,000 miles.
So you see, it's all subjective to your driving style. I can tell you on AVERAGE they last anywhere between 1-2 pads per rotor, but more aggressive driving will shorten that timeframe dramatically, depending on what sort of brake compounds you're using.