At what mm do you replace your brake pads?
Most brake pads start out with a pad width of 12 mm. As you brake while driving, your brake pads are wearing away as they create the friction against the rotors make you stop. When you reach 6 mm, that's the half-way mark of your brake pads like. You still have some life on your pads but should keep an eye on them.
When you get down to 3 mm it's time to replace your brake pads. As your brake pads mm get lower from here, the brake system is going to work overtime and can cause further damage to your brake system. You risk going metal to metal against the rotors or even worse, cracking your master cylinder due to the pressure exerting brake fluid through the brake system.