It all depends on the type of brakes you have and how hard you are on them. In stop and go traffic it is even worse. There is a sensor for the front brakes that scrpes the rotor and makes noise when they need replacement. If you have rear drum brakes you will not have this. One way to check on your brakes in the future is to note the master cylinder fluid level when they are worn out. When you replace brakes the pad or shoe thickness will increase the level in the master cylinder. So, in the future you can watch for the fluid level to reach the, "change"level when you get new brakes. Brakes can last as little as 10,000 miles or as long as 40,000 miles. The front brakes will wear out before the rear. The only real way to tel is to remove the wheel and caliper/drum and look at the pads/shoes and see where they are and measure them then check again in 1,000 miles and measure so you can know for sure how long they will las. If you are hard on the brakes, check more often especially in stop and go traffic use.