I've seen anything from 10k miles to 30k miles. I would probably stick with 25-30k unless I noticed a decrease in performance (then change sooner). My guess is that these things start getting pretty worn out past 30k - they're less durable than the expensive plugs, and besides, by 30k, they'll probably start going out of gap. I mean they're fairly soft copper just hanging out in a chamber full of explosions for like two years. I wouldn't bother re-gapping when you can get a new set for $12, and you're going to have to take them out anyway.
