The answer to your question really is based upon the primary drivers habits in braking, and driving conditions. A driver in stop and go traffic may replace sooner than one that has alot of highway miles. Typically, your front brakes wear twice as fast as your rear brakes, this may be anywhere from 50k-75k miles.
Brakes wear according to the driver of the vehicle. It's possible to get 30,000 miles out of a set of brakes or 10,000 if you ride the pedal a lot. You can have them checked when you take your vehicle in for service or you can check the level of the brake fluid in the reservoir under the hood. Usually when the brake fluid gets low it's time to service the brakes.
〉 Answered on Aug 16th, 2012 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines