I've always replaced NGK and Denso iridium plugs at about the mileage you're indicating. I start noticing a difference around the 60K mile mark and would never take them to 100K miles. You certainly might be able to do that in some cars but not in the three cars I've owned over the last 13 years that are spec'd for them. At least in my experience, the degradation in performance becomes less than subtle.