Depends on how you run it and maintain it. I've had rental John Deere 310 SE's not make it past 2 thousand hours with out blowing a head gasket. Then I've had 4 thousand hour rubber tire tractors with no real problems. Just hoses, oil, filters, etc. Depends on the operator. The tractors I run tend to last forever, I had one of the oldest back hoes in my company when every one else goes through one every 3 to 4 years. I made sure it was maintained and not abused.
I would say with good maintenance I would expect 4500 + hours in commercial use before needing $$$$'s or three to four years of constant use. Maybe two or more clutches in a utility type. A little less than an Industrial tractor. After four years you usually need seals and trans work from constant hard and heavy work. The companies I work for usually get rid of small tractors over 2.5K, because they can't afford down time caused by all the commercial construction use. I hate it when I see people slamming the shuttle in reverse and then back to forward while moving fast. Yes, popping wheelies is fun, but it destroys the trans.
I have an old Belarus with tons of hours, the meter stopped working at 6K and that was many years ago. It hasn't recieved the best care either. An engine should last at least 15K when maintained, but I, nor my family have ever kept one that long yet.
Anyone have one of those 15 thousand hour tractors? I saw an add for a ford / NH industrial tractor with 14K + on it. I wonder if a hydro trans will last that long on a smaller wheel tractor.