Generator hours - when is it too much?

As others have stated, I would want a genset that has been run (preferably consistently over its lifetime). Hard to be sure that was the case, but a 5+ year old genset with very low hours would likely not make my list. For high hours on a cheap genset I would be concerned period, but would likely draw the line at 100-200 hours if that could even be known. Quality gensets are another matter. If I believed the unit was properly maintained and run up to temperature consistently, then I would not be overly concerned with several thousand hours on the clock.
Commercial gas gensets that are run hard (long) easily last 10000+ hours.