From what I've heard, it depends on the car, the design of the shock, and the kind of use/roads.
I thought it was pretty standard that 3er shocks were only good for 40-60k miles. By the time my ZHP hit 60k, it was starting to be noticeable that they would need replacement soon. I'm sure a back to back drive with a new car would have made it extremely obvious they were worn. On my 190E, they were known to be good for 100k+.
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