Life Expectancy
Hi Joe, not only is the way the transmission is maintained and/or abused have a lot to do with it's longevity, but the transmission right from the factory can be a good one or a real poor one.
The pluses and minuses of mass production often add up to an occasional really tight tollerance trans and also a lot of really loose transmissions.
I've overhauled an original 235K trans, and it was in much better shape than one that had only 30K on it that I worked on a month later. It can depend a lot on luck how long your trans may last.
One of the main items done during a custom transmission overhaul is to attend to all the loose tollerances in a factory trans. These loose tollerances are the cause of internal pressure losses. If enough pressure is lost [leaks through loose tolerances] then there isn't enough pressure to hold the clutchs engaged and slipping results.
So there really isn't any way to answer your question accurately. It often depends on luck !!! But on an average, a hard worked stock trans maybe 50K, a good trans treated nice over 150K .
Greg L The Noise Nazi