What do you guys consider a lot of hours on a boat motor? Basically how many hours on a motor would keep you from buying it? 

Depends on the age. I would take a '01 motor with 200 hours over a '13 with 200 hours. But to answer your question, seems outboard motors usually last between 1000-2000 hours before simply wearing out to the point that they need rebuilt. Lower end for 2 stroke, higher end for 4 stroke. 

I guess there isn't a set number that would cause me to avoid purchasing a motor, I'd have to see a list of what had been done to it and take a look at the motor before making a decision to pass. 

I have a merc thunderbolt from 1976 that is original except fuel lines and filters and it is running strong as ever. Wonder how many hours are on it?

I asked the same thing on here even asked a boat dealer.  I don't think there is an answer to that     question.

Hours are not a good measure of relative health and most hour figures are not verifiable. A good marine mechanic should be able to give an opinion on the engine itself, fuel delivery system, lower end, etc.  I'd rather have a unit that had XXX hrs on it but was maintained properly than one that had just 50 but was neglected. Proper and regular maintenance is the key, a neglected unit will manifest itself.