Really no such answer and in the end would be useless info anyways.
I know guys like to advertise low hours but if they didn't do maintanence that low hours could be far more brutal then many hours. My son races and there really is no way to figure hours as one day he may practice on the 250F and only turn 5 laps on the 450 but the next day the 250F may not even come out but the 450 gets 5 hours on it.
If the seller is bragging about low hours ask how often (and when was the last time) he does oil changes, bearing greasing, topends, clutches.......
If he hesitates or has a blank stare I'll bet he has no clue- if you ask me I can tell you exactly what our schedule is