I believe that my Yamaha manual's service recommendations equate 500 hours to a year's use in the same way that cars average 12,000-15,000 miles per year.

I've put 700 hours on my main (only) in the last 4 or 5 years, all in the salt, although there are plenty of times where it is just idling. I was trying to calculate all the time I'm out there without it running and figured I must be living alright.