I change mine every year. It may be overkill but I had 1 go out a few years back that was only about 18 months old.
Good thing about the 496 is that it will shut down into safe mode if it gets to hot.
My 7.4 is old school Merc with no computer. It didn't shut down and the impeller failer cost me about $5000 to fix everything.
So for me, I make it part of my yearly service. $35 for an impeller is cheap for some peace of mind.
The 496 impeller is a little more involved to change from what I recall, but not rocket science.