On a W210 the minimum pad thickness is 2mm. Terry's values include the backing plate. The wear sensors (if you have them) are 3mm thick so that's the point they'll go off giving you a little time to have the brakes serviced. I don't have the values for your car but they would be in a factory service manual. If you post in a W124 subforum such as on www.benzworld.org, I think you'll get the exact values.

If you want you can pick up a cheap micrometer at Harbor Freight for about $20-30 which will be more than accurate enough. You'll have to convert from inches to metric at some point. (1/1000th of an inch = 0.0254mm) Check each rotor at 4 positions about 90 degrees apart and average. Check the pads at two positions in from each end and average.