This all depends on your driving habits. If you race from red light to red light, or always have you foot on either the gas or brake, you can wear them down(to need of replacement) in 15,000 -30,000 miles. If you drive more conservatively, you can get better than 100,000 miles of a set of brakes. My F-150 has 102,000 miles and I've still got better than 50% of the pads left.

Modern cars have a brake pad wear indicator. When your pads need replacing, the wear indicator begins to drag against the rotor and make a grinding noise when you press on the brake. This will alert you that your brake pads need replacing. The wear indicator sounds before the pad is completely gone.

Better, when you have your tires rotated during oil changes, ask them to check the brakes. They'll look at the pads to see how much they've worn down.

Its "brakes" and has been said it can be very variable. My car is on 78,000 and on the originals

Brake. Anywhere from ~7,000-20,000~ miles. Had an H2 in a shop once, hadn't changed the rear brakes in 70,000 miles. Still good too.

It depends on the mileage of the car, my mileage is around 25,000 and brake pads are still ok

Anywhere from 30k to 100k miles depending on driving style and conditions.

thanks I have done only 15000 so no need to worry just yet.