Well, so far in seven years of driving my MBs I've not yet had to replace the pads twice (all except the 300D had terrible brakes when purchased). One set on the Volvo in 50,000 miles.
I expect around 30,000 or so on a set of pads, that means around 100,000 on rotors for me.
I do almost exclusively "open road" driving at moderate speeds and rarely have to brake hard. If you drive in heavy city traffic with lots of hard stops, expect considerably less. If you do almost all interstate, expect more.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!