IMHO, something like my tillers which may get 5-6 hours use per year have the original plug(s), cleaned as part of fall tune up. My GT which mows four acres and pushes snow in the winter get's new plugs as part of fall tune up.

Going back to the old days when we tuned up our vehicles, "points, plugs, condenser, filters. Anybody ever have a vehicle run better after a "tune up?"

I remember the Old Days Well and Running Better (my 1968 and
70 VWs) and almost a must. All before Electronic
Ignition and I have not seen a yearly need since with Electronic Ignition. The plug or plugs are the only wearing items IMO and I do not see the wear we saw back then.
I look for wear as I clean and CK the plug’s gap.

Performance is my gauge. Such as, starting 1st pull (if
primed properly), consistent gas usage, same good power and no signs of oil
usage, “Every Start”!

I don't change them unless I have problems not explained by something obvious. Admittedly, my skills may not be up to some others here. I got my initial experience from the VW Idiot's Book with our 59 VW Bug, 65 Squareback and 68 Bug. I rebuilt the '59 engine. I had an old Ward's 14" chainsaw that I ran for 25 years. I guess that I may not have had its tuning set to best advantage because I tended to get fouled plugs with some regularity, but I just had several and cleaned them up in batches and reused them.

My skills, levels of interest
and time can not compete with many. I
feel I fit with most posters and enjoy the sharing. I wish I had their time, patience, equipment
and space. We all seem to adapt to what
we have enjoying challenges and move on to the next task. There is always another Task!!! Our Travel though the experiences have been
too quick. Take Care....