That's a good number of hours, in my opinion. It indicates the boat has actually been used on a regular basis. Much better than it sitting around all year rust, corroding, mildew-ing, etc.
It used to be 250000 miles and you new it was time for a new motor for your truck/car, but a boat is more like a generator, and the hours you have on it?
As long as you have done the regular servicing on them I would expect that and THIS IS JUST A GUESS BUT 4500-6500hours before any major repairs are required in a perfect world!
If a diesel jetta can get 3/4 of a million miles before the cylinders are worn out a gas marine/ genny should have no problem getting to that point!
just my opinion.
rob