In a 4 cycle lawn mower--10 years at about 1 hour per week
for a minimum of 25 weeks. In a 2 cycle chain saw-- 8 years
of actual run time of about 16 hours per year. Nothing
should wear out in 8 hours of use, but something could
break. Most likely it's corrosion of points, switch, etc.
and repeated start tries wears the corrosion until good
electrical contact is made. BUT, it could be failure of an
electrical part.

Peter wrote:

My lawnmower won't start again.
It is a four stroke Honda, the fuel supply is OK, the spark looks OK
when I take the plug out and rest it against the cylinder head and
pull the cord.
However, I know from experience, that as soon as I go down and buy a
new spark plug, the mower will work again. The plug seems to need
renewal, even though I have a small lawn, and it would have done no
more than 10 hours work, maximum.

What life do others get out of their plugs? Could I be missing
something?
Ta
Pete