Most vehicles you get 40-50k original miles on the first set of brakes. Depends on your driving. I would think very hard driving and lots of track use you would have to change them at about 35k.
As long as they are functioning properly, I don't think there's any set mileage to when you need to install new brake pads. It's really a matter of how much pad is left. I want to say that a good rule of thumb is to change them when they get down to about 1/8" thick.