The rotors are considered "sacrificial" meaning that they are never machined, just replaced when the min thickness is exceeded. Usually 3 pad replacements to one set of rotors. Fronts wear 3 to 4 times faster than the rears. Thats why the wear sensors are only on the front pads.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.