It's more about time than mileage, and MB mounts usually go for about 5 years before they start to show signs of compression. Ours is 5 years old, 90k on the clock, and I felt a slight rumble under the center arm rest the other day when it shifted into a different gear, a clear sign of the transmission mount compressing a bit too much. My W212 did the exact same thing at about year 4, but on that car (2014 E350), one of the problems was the engine heat and proximity to the mounts. The heat accelerates the softening / compression. We're probably getting close to the engine mounts too, judging by differences in engine sound in the cabin. I can just "feel it". Not quite yet, but I can tell their replacement isn't to far off in the distance now.
Honestly though, I've never actually seen an engine mount "leak". We have no signs of leaking anything on ours. I've heard of engine mounts leaking, but I have never actually seen one that did, and I've always asked to see the old mounts when pulled from the car. Always been solid compressed rubber.