In my experience brake pads last between 30,000 to 40,000 miles but they can also last a lot longer depending on how often you use the brakes to stop the vehicle.
It's the amount of stopping you do that wears the brake pads out and not the amount of miles you drive the vehicle.
After 30,000 miles it's a good idea to have the brakes inspected to make sure they're in good condition because brakes are very important to stop your vehicle.
Brake pads also contain a metal strip that scratches against the brake rotor when they wear down enough to need replaced so if you hear a scratching squealing noise when applying the brakes then it's likely the brake pad wear indicator letting you know the brake pads need replaced.