Most brake pads last between 40,000 to 80,000 miles before they need replaced.
The only time brake pads wear down is when you actually press on the brakes to stop the vehicle so when you're rolling down the road the brake pads are not being worn down.
So the amount of stopping you do with your vehicle will determine the life of the brake pads as the more you stop the more the brake pads will wear down.
In my experience with a lot of highway driving the brake pads last on my vehicle for at least 80,000 miles but after 80,000 miles I usually replace them regardless because it's pretty easy to do and brake pads are so cheap.
I pay at most $40.00 for a set of front brake pads for my vehicle so it's cheap insurance to know that the brake pads are in good shape.