i dont think there is an actual standard on how many miles you get. it all depends on where you drive(city, highway, hills with big downgrades, or long open areas), how hard you use the brakes(do you wait until the last minute to brake or do you start braking with plenty of time to stop), and how often you use them(again highway or city driving).
the best suggestion i can give is that when you go to get your tires rotated have them look at the brakes.
you can do it yourself if you would like. here is a good article that explains the process and inspection of the brake pads.
http://www.edmunds.com/ownersh....html
when looking at the pads, if you see giant chips and gouges in the pad, you may want to remove them and fully inspect them to see if the pad is failing
p.s. on most cars, the front brake pads will normally have to be change long before the rears ones do. the rear pads are mainly used for the parking brake and do not wear as fast as the front ones.