That is a highly subjective question. Kind of like the drunk where 1 drink is too many, but a 1000 is not enough. I'm sure it's possble to wreck a tractor in a few hours, yet I would expect a good quality Tractor (Any of the big 3), to go several thousand hours if properly maintained. That's the key, maintenance, and usage patterns. Some of this is discernable with a visual inspection, some is not. I've seen machines that looked as if they had been through a war, but were still fully functional. If you have any doubt, get a qualified mechanic to inspect it for you. It's surely worth an hour of his/her time if you think you are going to for it.