Brake pads should last from 30,000 to 40,000 miles as long as you don't constantly ride the brakes or do a lot of stop and go driving which can cause brake pads to wear out more quickly than they normally should.
Other things that could cause brake pads to wear out more quickly are Seized guide pins, bushings and slides can cause the brake pads to wear out quicker.
Warped rotors or rotors that have grooves in them could also cause brake pads to wear unevenly or wear out much quicker too.
If your brake pads are wearing out too quickly and you're not doing a lot of stop and go driving or constantly riding your brakes then you should have a brake shop or other automobile shop check the brakes out to fix the problem.