It depends on how a person drives a car. If they ride the brakes a lot or do a lot of stop and go driving then the brakes can wear down as soon as 25,000 miles.
But normally the brakes will last up to 70,000 miles or more. You should always inspect your brakes and look for worn brake pads before it becomes a problem.
Never wait for your brakes to rub metal to metal as it can damage your brake rotors and cost a lot more to repair than just replacing your brake pads.
And remember brakes are what's stopping your car. So they're very important part of your vehicle.