How often you should need to replace your brake pads depends on how much stop and go driving you do and how often you use the brake pads to stop the vehicle.
Most vehicles only need the brake pads replaced after 50,000 to 70,000 miles and some vehicles only need the brake pads replaced after 100,000 miles.
When brake pads need replaced the wear indicator on the brake pads will start scraping against the rotor which is an indicator of brake pads needing replaced.
Some shops will inspect brake pads for free while others may charge a fee but it's well worth it to determine if your brake pads are in good shape.