What he said...I've yet to change my plugs from the factory...engines today are not like they were 20 years ago, and plugs are better today too.
I've checked them at 100 hours, and at 195 hours as of this date, they're still good. The manual calls to check them every 100 hours - there is no requirement to change them at 100 hours or yearly, just to check and change them when they're worn or there's an issue. If you think you must change them every year, well that's just marketing BS to sell more plugs. The reason they recommend to check every 100 hours is simple - if you run WOT all the time, then they're probably showing wear. If you idle and cruise a lot like most people, then they're good for many more hours, but they need to look at the worst case scenario to recommend check and replace. It doesn't mean your use requires the same schedule.
Having said that, I'll probably change them out mid-summer next year and see if it makes a difference. But since I'm still getting 7600+ RPM at WOT and 53 MPH, I'm guessing they're still good. Three full seasons of use seems heavy to some, but since my average is below half throttle, I don't expect to see much of a difference when I change them.
If you compare to vehicle plugs, then ours should last for 25k "miles" or more in the same conditions and plugs, so in our boats that means about 5+ years of average use. That's the schedule I followed for many years of boating....not much has changed in the last few years.
I check my oil every weekend of use for issues. If everything is good, I can usually get 2 seasons out of my oil - no reason to change oil every year, my oil is clear amber in colour which indicates good conditions. I'm not gentle with my use either...we have lots of no-wake speeds, lots of cruising, and lots of WOT. To say oil must be changed every year is a myth to sell more oil, just like spark plugs.
Yes there are situations where heavy use wears the additives and viscosity down, but unless you're doing WOT 95% of the time, it's just a selling feature for the manufacturers. Remember, they're not in it to help you, they're in it to make money.