I want to get an idea how long you guys have gone before changing out your brake pads and rotors. I'm coming up on 21,000 miles and am thinking they might need a changing soon.

 

22,000. Rear pads are still ok but front needs done soon. My Lyndall Z-plus pads just arrived yesterday.

 

Like tire life, it's pretty hard to put a mileage number on brake life as it's very dependent on the type of riding you do. Lots of stop and go city riding or hard twisty running is much harder on the brakes than highway cruising where you might stop only once every 150 miles for fuel.

My rear stock pads were shot at 10,000 miles, the fronts at 14,000. Replaced the stock pads with Lyndall's which last 3 times longer, stop better, no brake dust and no brake squeals.

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At 21K miles, you should NOT need to change out the rotor. I haven't changed out my rotor in the 12+ years I've had mine.

 

+1 on that, you should not need to replace rotors unless you grind the pads to nothing and they damage the rotors.
As others have said, mileage is not the best indicator, inspect them, they are easy enough to see, if you have less than 1/8" of pad on either side, replace them.