The rotors on Mercedes are a softer metal than we in North America are used to, and they wear out at an amazing rate. My little car (B170) needed new pads and rotors at 30K miles, and the tires were at the point I decided to replace them with winter tires in October.
I do live in the mountains so there is that factor, but still only 30K miles is pretty fast wear.
Sounds about right. In the "old" days I would pop these on in 30 minutes each.