If you look at it from a materials standpoint, you can make a case that a particular transmission is designed to execute X number of shifts for it's design life.

Factors modifying the X number would be, of course, how hard the shift was (light throttle, hard throttle) and the amount of torque transmitted (engine power). Fluid changes to remove the "fines" that accumulate from wear products would be beneficial, more so if the transmission is driven hard.

So, lifetime will vary considerably depending upon use and maintenance.

Obviously, the trans that is driven in town, under hard acceleration will have the lowest lifetime, probably independent of fluid changes. And a small engine car will tend to shift much more than a lazy big engine car, giving the big engine trans a better chance at a longer life (if not abused by excessive power application).

And the longest lived one, will be one that lives on the highway, accumulating hundreds of miles between shifts. Add in a light footed driver, and possibly unnecessary (premature) fluid changes and you have the recipe for potential high mileage.

My 91 300D belonged to my cousin Tim, and his use followed the above, The 300D currently has 301K on it, and the original trans is quite nice, firm, crisp with little or no lag into reverse. Likely good for many more miles.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles