The brake pad is a simple, but very important part of the entire braking system of a car. Since brakes are obviously a crucial part of a car, especially when it comes to safety, overlooking the brake pads may result in accidents with the brakes not working when they need to.
To understand why brake pads wear out, it’s important to have an overview of the braking system in a car. In a nutshell, when the brake pedal is pressed, a brake fluid passes through the thin wire called the brake line. At the end of this brake line, this fluid applies pressure to the brake callipers (which holds the brake pads in place), and the callipers press the brake pads against the rotor (which is attached to the wheel and rotates along with it).

Simply put, the brake pad is what is being pressed against the rotor to create friction to stop a car. This brake pad has a steel plate on one side, and friction material on the other side which will face the rotor.
Now that we understand what the brake pads do, let’s see how to detect wear and tear inthem.
To detect anything amiss with the brake pads, you just have to keep your eyes and ears open.

Visually examine the brake pads- Just take a look at the brake pads through the tire spokes. What you’re looking for is the thickness of these pads. If the thickness seems to be less than 1/4th of an inch, get them inspected. There’s a good chance that they need to be replaced.

Indicators on the dashboard- Some vehicles come with indicator lights on the dashboard which tell the driver when the pads have worn out and need to be replaced. This is the easiest signal to watch out for.

Screeching Noise- Often when the brake pads have worn out sufficiently, a small metallic plate called a ‘shim’ (embedded in the pad itself) will give out a screeching noise, which is loud enough to be heard even with the windows shut. Continuously hearing this noise is an indication that the brake pads have been worn out and need to be replaced.Take note that such a sound can be heard even after the brakes have been exposed to any wet conditions, like rain. In this case, however, the screeching sound won’t remain after a couple of stops.

Deep growling noises- This would indicate a definite and serious sign of the brake pads being completely worn out. This low, rumbling noise indicates metal to metal contact – that is, the rotor/disc is grinding against the brake calliper. Do not let this situation persist. If this persists, even the rotors could get damaged.

Brake pads generally last around 30-35k miles. But it depends on the driver, the car, and the traffic conditions. A car which is driven in a congested city will need to use brakes far more often than a car in a rural area, leading to more wear and tear of the brake pads.