Once the timing is set, I'd say it shouldn't need checked but only maybe every 150 hours. You could make it part of the tune up and check it when you change the spark plug. But only need to do that on about the same interval. By true hours of operation, 150 hours isn't that many, but most people it takes 2-3 years to put that many on a tractor. Pull it every few years, if it looks good, clean it and put it back in. When it shows wear, replace it. I'm not one of those guys who thinks they need replaced before they are worn out. I don't remember the last time I even had the plugs out of any of mine.
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