Having farmed most of my life, and had anywhere from six to fifteen tractors at any given time, I can say with some confidence that, if maintained and not abused, the average tractor will function with no major repairs for at least 4,000 hours, most go to 6,000 hrs before an engine overhaul. Years ago I bought a new Romanian made, Universal, 60hp tractor (considered a throwaway) that had 10,000 hrs on the clock when I sold it and it still ran flawlessly.
Occasionally a person gets unlucky. We currently have an 80 hp Mahindra that spun a bearing and had to have the engine rebuilt at 1,900 hours to the tune of $6,000 for parts. Needless to say we'll never buy another one of that breed (we rebuilt a comparable hp Ford with 7,200 hrs on the clock, the year before for $1,400 in parts).