Brake pads usually last between 50,000 to 100,000 miles depending on how often you use your brakes to stop the vehicle.
If you're constantly driving in stop and go traffic and constantly applying the brakes to stop the vehicle then your brake pads will wear down faster.
Also if you ride your brakes a lot that can also cause your brake pads to wear out sooner.
Some indications of brake pads that are worn and need replaced are a grinding down when you press the brake pedal.
Or your brake pedal goes to the floor.