The OEM service interval for both "normal" and "severe" driving conditions is 105,000mi. I couldnt IMAGINE running on a single set of plugs that long. When the car is N/A I would recommend pulling and checking the gaps every 30,000-45,000 miles and replacing as necessary. For a boosted car I would plan to REPLACE every 30,000 - 45,000. This will keep you running at peak performance and prevent misfires and detonation from worn plug gaps.
Just my .02
Thanks,
Alex Goodwin
AlexG@motionlabtuning.com