Hmm, i'd have to say the life span varies on the type of use the owner has in store for it and how long it's been sitting prone.... time and/or excessive stress on the engine are surefire ways to lead to a quicker ending....
In my case, time was the true killer.... my TII only has 97K, yet it's got 80 on the front and 60 on the rear... (just to note, if you already didn't know, a perfectly healthy rotor is 120 PSI on each side.... 90 or higher is acceptable.... anything less, time to start saving pennies) It had sat in a lady's driveway for a full year, flooded and drained of power no less...
I'd ask the person selling it about maitenence history, and for god's sake, bring along a compression tester.... lol, unless you plan to do heavy engine work on it anyway.... Also, if the engine refuses to start up or even turn over, i'd look elsewhere... totally not worth your time, due to who knows what internal damage lies within....