There is not set mileage. If you do alot of city driving, it could be as soon as 10,000 miles. If you drive nothing but highway driving, it could be 100,000 miles.
I think the official thickenss is 3/16". When the friction material gets that thin, start looking at changing the brake pads.
When that occurs depends upon your driving style, the terrain, and the amount of city/highway driving you do. Ford recommends checking your brake wear every 10K miles But I find it makes more sense to peak at the brake pads anytime the wheels/tires are removed. And since I rotate tires about every 6K-7K miles, that's a perfect time to check.
