I usually don't check them until I think there is a need to. Misfiring,
stumbling and hard starting not traceable to any other cause are normal
symptoms. I would say that those who are not experienced in doing so,
especially in engines with aluminum heads, probably should err on the side
of leaving them alone rather than messing up a threaded hole.
I've put about 52,000 miles on a 92 Taurus since I first started driving it
in early 2000 and replaced spark plugs a couple weeks ago. To the best of
my knowledge the plugs were a year old or more when I got the car. My guess
is that the wires were worse than the plugs. Older cars with less accurate
mixture control or oil consumption problems may need either more frequent
attention, hotter plugs, or both. A rich mixture is bad about contaminating
plugs with soot, while a lean mixture tends to overheat them. If they are
"too clean", snow white insulators, etc, that is a sign they are running
too hot, too lean or both.
--
George Downs, Bartlesville, Oklahoma, Central US