Front brake pads should last between 30,000 to 50,000 miles depending on how often you use your brakes.
The more stopping you do with your brakes the faster the brakes will wear out.
If you do a lot of stop and go driving in town then your front brake pads may wear out as soon as 20,000 miles.
But if you do a lot of highway driving and use your brakes not that often then your brake pads will last longer and can last 30,000 to 50,000 miles.
After 20,000 miles it still would be a good idea to inspect both your front and bake brakes to check for how worn down they are.
If they are getting thin then replace them to be safe.
If you hear a scraping sound when braking then the brake pads could be worn down because brake pads have a wear indicator that rubs and scrapes against the brake rotors when the brake pads are worn down.