See Also Brake Pad life and Brake Job Cost

That slot goes down to within a couple of millimeters of the bottom, so when it disappears it's time to get new pads.
Some newer cars have a sensor on the pad which will cause a warning light on the console when they need to be replaced.
The wear indicator slot on disc brake pads is usually two millimeters deep. Brake pads with sensors typically begin to warn the driver when they reach a thickness of 3 millimeters, or 1/8 inch, with a warning light or a squealing noise.

Rotors are usually replaced with the pads, but technically if the rotor is not worn down or warped they don't have to be replaced. They used to resurface rotors, but I don't think that is done much now.

The inner pad will always wear quicker due to having more pressure applied directly from the brake piston.

The front brake is larger due to weight shift during braking.

last updated 25 Apr 2024

I got 48,000 on the original brakes on my 2020 Subaru Crosstrek with mostly highway driving.
I saw conflicting information on whether they were ceramic. Many web sites indicated most OEM brakes were Organic, but they seem to be shifting to ceramic. one site said that in 2024 90% of OEM brake pads are ceramic.

Organic non-metallic (NAO) :
Most volume manufacturers and premium manufacturers fit their cars from the factory with Organic Brake Pads, but this only refers to over 50% of new vehicles.
These pads do contain between 10 to 30% metal, but the rest of the friction material comes from plant-derived fibers, high-temperature resins, and other materials. Depending on the proportion of metal in them, they might also have Kevlar, carbon, rubber, and even glass in them.
Semi-Metallic Brake Pads
They feature from 30 to 65% metal by weight. Multiple types of metal are used, from copper and iron to steel. The rest of the friction surface is made of fillers, modifiers, and other substances required to enhance performance and maintain reliability.

Before the appearance of ceramic brake pads, semi-metallic pads used to be the best performing pads available on the market.

Semi metallic stop better than ceramics. But they have to heat up to provide the best braking. They have better fade resistance than ceramic.

Ceramic brake pads are the latest inventions in the world of brake pads.
Ceramic brake pads are made from a mixture of clay and porcelain, reinforced with metal filaments, flakes, and other materials. The main component is alumina, and they also contain fillers without iron and adhesives.

Akebonon ceramic pads are recommended a lot.
Carbon Fiber:
Carbon fiber pads are the newest (and most expensive) type of brake pads. As you might guess, these pads contain a carbon mix with embedded fibers. You’ll usually find carbon fiber brake pads on race cars, air crafts, and other high-end applications.
Brake pads and rotors should be replaced in pairs. New pads on worn rotors can cause vibrations, noises, and poor stopping performance. As rotors age, they can develop a hardened surface called "glaze" and wear patterns that don't match new pads. This can lead to early deterioration of the new pads and require another trip to the shop.

New pad break in procedure.
To ensure best performance and maximum rotor and pad life, it's important to break in your newly installed pads and rotors. Ideally, this would be done on a deserted road, with multiple stops from 35 MPH to 5 MPH in succession two or three times, with a cool down period afterwards - normally a drive home. It's possible to do this during normal everyday driving as well for the first few days, increase your following distance so that any braking done can be smoothly and gradually, decreasing pedal pressure as the car slows to a smooth stop. Avoid panic stops as much as possible, but this is a good rule in general.
Tech notes
Why are Front Brakes Larger than Rear Ones? Brake Bias Explained | 15 min YouTube - Good technical explanation of friction, inertia, force, ...
Brake Pad life and Brake Job Cost
Brake Pads: Organic, Ceramic, And Semi-Metallic - What Are The Differences
Ceramic vs Semi Metallic vs Organic Brake Pad Comparison | YouTube - First Place Auto Parts

How Long Do Brake Pads Last on Average? Ceramic vs. Semi-Metallic Brake Pads: What’s The Difference?