depends on riding style. I am only on my 2nd set in 15,000+ miles and just changed them at about 13,000 give or take.I use my engine to slow down a lot and tend to break equally between my front and rear, the front having most of your stopping power. On the other hand it could be a caliper sticking or a warped rotor. (probably not a warped rotor unless you feel a pulse or vibration on breaking. You can also look for a discolored area where the rotor has gotten hot, but doubt thats the issue)If you tend to use your rear break by itself to slow or just use it more to stop than the engine or front then thats why.
