Q&A

 Master

Ask why. They should be able to tell you how many millimeters pad material is left. Ask to be shown, if they get huffy, there may be a little funny biz going on. Not uncommon.
Some talk in percentages but ask for mm's. 1 mm is definitely time to do the deed. They are at least 10mm thick when new.
Some brakes wear out quick, some last maybe 85k miles. Depends largely on driving habits and materials. If the brakes shudder when applied, the discs (rotors) are warped and need machining or replacement (and new pads). If the pads are worn down badly, it's time for pads and preferably machine or replace rotors. I worked MBZ and BMW many years ago and the pads wore quickly but you could reuse the rotors without machining 2 or 3 times. It was due to using soft pad materials which give good performance but don't last too long. Not bad. I don't know if the newer brakes can be treated that way anymore. I think probably so.
Maybe you'd like to find an independent Mercedes specialist. Here's a directory link for you:
http://repairpal.com/directory?address=32771&car_brand_na...