Your car's brakes use a great deal of friction to bring the vehicle to a stop, and the brake pads are designed to act as a cushion to protect the calipers and rotors from this high-impact action. They are also designed to wear down over time. Whether you have ceramic, composite (mix of metal and other materials) or metallic brake pads, they will need to be changed periodically.
The life of your brake pads depends largely on how you drive. If you constantly slam on your brakes, you're going to wear the parts out much faster than if you have a more gentle approach to stopping your car. In either case, you should manually inspect the brake pads on occasion to ensure they are not worn past the point of safety. This can usually be done without even removing the wheel. However, in some cases, you may need to remove the wheel to get a better look. If you're not sure how to do this, have your mechanic take a look next time you get the oil changed or bring the vehicle in for servicing.
A good rule of thumb to follow is to replace the brake pads if they are less than one-quarter of an inch thick. You can still drive the car a bit longer when you're down to that much material, but it is imperative to take care of the replacement as soon as possible. If the pads are less than one-eighth of an inch thick, you'll want to replace them immediately.
Since many drivers do not take the time to inspect their brakes or may not know what to look for, most brake pads have a built-in warning system. A small metal rod called a wear indicator is hidden within the pad material. Once enough of the material has worn away, it will expose the wear indicator. When you apply the brakes, the rod will come into contact with the rotor and produce a high-pitched squealing sound. If you notice your brakes are screaming, this is their way of telling you to get the pads changed as soon as you can.
By keeping an eye on brake pad health, you can ensure your car's stopping mechanism is in working order. It is also a good idea to examine other components of the car's stopping mechanism for signs of wear or damage while you're checking the brake pads.