i used to have a 2.0L nissan motor with 25psi of boost on 93 octane. that motor would average about 2 months with copper core NGK plugs. I ran Denso iridiums in the late 90's in it and would average about 4 months. when NGK iridiums came down in price i started using them and they lasted about 4 months aswell. If the engine is pretty stock and not a high performer i have had great luck with the $1 NGK plugs. like my c10 pickup with stock 350. going on 20,000 miles no probs. my cb750 had NGK iridiums in it for about a month. they were a stop colder and didn't last for shit. i tried copper NGK and they lasted about the same. i stepped to the stock heat range and got an extra month out of them. so now i just run the cheapos in it and expect every 3 months to change them.
but in high power applications, like my nissan (500whp) it was worth it to run the iridiums. cured all of my high boost missfires. its really a thing where if you want to spend the money, and don't like changing plugs as often, do it. but the coppers are so cheap, i just buy boxes of them and keep them around.
Hack, you're a smart cookie, what do you think about that link to those capacitor plugs a few posts up?