Disc brakes need to be changed when the brake pad is worn down and begins to rub against the brake rotor, resulting in a squealing noise. The noise is caused by a wear indicator attached to the brake pad.
Average drivers should have their brakes checked every six to 12 months, and frequent drivers should have their brakes checked every three months. Visual indicators that brake pads need to be replaced include brake dust accumulation in the wheel and circular marks on the brake rotors. The brake pad is visible through the wheel on most cars and should be replaced if the pad is extremely thin.