Properly maintained I've seen several 12k+ hour champions with the 8 speed trans that I know are original. I'm sure with proper maintenance they probably go further than that, but I've seen 12k.
I had a relatively low hour Dresser 850 for a few years, (5500 hrs) it was a nice machine but when I went to sell it I had a guy look at it that knew them pretty well and there was something about those transmissions that needed an update or something and he didn't have much interest in performing it himself eventually, had it already been done he wasn't afraid of it. That machine was a '92 I believe. I've seen similar Dressers with 8-9k hrs and all original, you never seem to see them with crazy hours though, like 17-18k.
This is all anecdotal from my observations looking at these same machines, hopefully some guys with more technical knowledge chime in.