I drive almost entirely highway miles. My 240D finally got two new rotors, actually used from a parts car, on the front at 518,000 miles. I can't remember how many sets of pads I've replaced, I'd have to look at my logbook. I would guess about five sets.
As long as the rotors aren't warped causing pulsation in the brake pedal, or they haven't had pads worn to the metal, or very thin, I don't think you need to worry about replacing them. I may very well get corrected by some other members on this, but that has been my experience with rotors on the 123 cars.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in