The factory plugs are 100K-mi maintenance interval. I'm replacing mine at 72K because it's starting to chew coils. If you replace a coil you replace the plug. One seems to make the other go bad on these things.
With copper plugs you need to check them every other oil change. They don't wear as well as the others. Manufacturers started using platinum and iridium plugs mainly because spark plugs became such a PITA to change. They wanted something that would out-last their new 60K and 100K mile maintenance warranties. They didn't want to pay the dealer to disassemble the upper end of the engine in order to change the plugs. Anyone who has done an LS V6 COP/Plug knows what I mean.
Iridium is the 'better' option compared to platinum. But it costs you another $2/plug. That's why the manufacturer (bean counters) went with platinum plugs. Iridum has better conductivity and can transfer heat better then platinum. This means you get a hotter spark with less of a chance of over-heating the plug compared to Platinum. Iridium is also a lot harder then copper or platinum which allows it to have a finer point on the electrode. This helps to concentrate the spark in one spot instead of it dancing around on the plug.
Copper is the best performing plug, and the worst longevity.
Platinum is the worst performing plug, with good longevity.
Iridium is a good performing plug, with the best longevity.
