Not really a simple answer. When I bought my boat it had a mere 20 hours on a complete rebuild with new heads. Both motors didn't hold together for 10 hours under light use. All depends on the builder and boater. I've seen stock and high performance motors last from 30-1000hrs.
IMO - rebuilds at 500hrs on a high performance application aren't a bad idea. When buying used the best you can do is a leak down and oil sample test.