You really only need to change brake pads when they're worn out.
Your brake pads should have a little piece of metal that will make contact with the rotors when you apply your brakes.
If you hear that scraping or squealing noise then it's time for your brake pads to be replaced.
Brake pads usually need replaced at 70,000 miles but some might need replaced sooner if you do a lot of stop and go driving.
The less often you have to apply your brakes the longer your brake pads will last.