Brakes on any car wear differently from driver to driver. Most cases you should be able to expect at least 30-40,000 miles out of a set of brakes. There are also a number of other factors that can cause premature brake wear on any type of car. The brake caliper slide pins may not have been lubricated properly when the brakes where intially done. This will cause the brake caliper to sometimes stick and cause premature wear and can damage the brake rotors prematurly as well. Alot of factors go into getting the most out of any brake job. The type of brake pad material that is being used is one main factor. Brake pads for any car can be purchased as little as $20.00 or as much as $85.00 for the same car and application. Ask what happened to make the brakes wear in just 20k and the shop should be able to give you some sort of explanation.