How often to change plugs?

[aquadj]

I have a 2006 Yamaha 250 4 stroke. When I had 2 stroke motors, I always changed plugs in spring after the winter layover, but is it necessary with 4 strokes? I don't fog this motor by spraying through the intake while running, I spray fogging fluid into the spark plug holes and crank the motor for a few seconds without it starting. Since I last changed these plugs I've put 110 hour on.

[rodbolt17]

we typically do it every 100 hours, however I have seen them go 500.
and your fogging it wrong.
wife is out of town this week, ill see if I can make time to do some posting.
its just a bit busy now, cold weather winterizing and such.