When the light comes on, I go ahead and inspect the pads for wear, and find that there is still plenty of pad left before the danger of metal-to-rotor contact. Based on my pad change intervals, I measured the thickness when new, and when the sensor does it's thing, my estimates come to around 1500-2000 miles...note that I'm am easy driver...autobahn-like tendencies will shorten pad life and expected time to replace, dramatically.

I am also making these statements with my pad replacement experience on my W124 (which have the same basic brake setups, btw).

I bought the R129 in 2000 and have YET to see the indicator come on that one (yeah, I did check the pads and sensors).

Note, that MB engineers a "sloth factor" to all of their components, so that lackluster owners are not severely "dinged" by not following strict maintenance routines.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car