When your brake pads get worn down to a thickness of between 1/8" to 3/16" and if you hear the wear indicator scraping against the rotor when pressing down on the brakes that means the brake pads need replaced immediately.
Depending on how you drive and whether you do more highway driving over stop and go driving brake pads last between 70,000 to 100,000 miles before you need to replace them.
If you wait to long to replace the brake pads it can damage your brake rotors and also cause a dangerous situation meaning you may not be able to stop the vehicle.
Some places will do a free brake inspection so look for some place that may do free brake inspections.