Benz rotors need to be replaced every third pad change (that is, you
can run two sets of pads on them). They cannot be turned -- by the
time you get them smooth, they are almost always too thin, and this
will cause the pistons to seize in the calipers before the friction
material is worn out on the older dual piston calipers. Not good....
Newer models go further, but on the W124 and earlier models, you can
expect new pads every 30,000 miles or so depending on driving
conditions and new rotors every 90,000 in the front. Rears should last
at least twice as long.
Peter