When you purchase a new brake pad, it has a standard thickness of around 8 to 12 mm or about a half inch. Over time, your car's brake pad will wear away. A brake pad at 3 mm or around one tenth of an inch will take around 40,000 miles of driving to completely wear out. Don't wait until then to replace your brake pads. Once your brake pads reach 3 mm it is recommended that you schedule an appointment to get them replaced soon.