The pad thickness should be inspected every time that the tires are removed for rotation. The outer pad has a wear sensor which will make a squealing noise when the pads wear to the point required replacement.
When the pad thickness is worn to within 1 / 32 in. (0.76mm) of the shoe or rivet, at either end of the pad, replace the pads in axle sets. This is the factory recommended measurement; however, your state's automobile inspection laws may not agree with this.
When checking the disc brakes, check both ends of the outer pads by looking in at each end of the caliper. These are the points at which the highest rate of wear occurs. The inner pad can be checked for premature wear by looking down through the inspection hole in the top of the caliper. Some inboard pads have a thermal layer against the shoe, integrally molded with the lining. Do not confuse this extra layer with uneven inboard/outboard pad wear.