In general ;
About 250,000 miles .
*IF* however , you do foolish things like drive it with burnt up old fluid in it that smells like Navy Coffee , or sip it into drive whilst the car is still rolling backwards (no matter how slowly) , expect much shortened Slushbox life .
This is why so many American Drivers complain of trannies dying before 100,000 miles ~ they're simply not designed to take this sort of rough service .
Revving it above idle whilst waiting for it to take up , slamming shifts up or down , these are all things American Drivers do on a regular basis , I see it daily .
My Mercedes are all older ' beaters ' I got mused with lots of miles , I never ,
EVER abuse the trannies and have gotten well over 350,000 miles before needing rebuilding .
My '82 240D's tranny has been on it's last legs since I bought it over a decade ago but SWMBO doesn't want to $pend any $ on it so I keep a sharp eye on it and replace the fluid & filter annually . when it goes , she'll have to get it fixed .
-Nate