I did mine ('01, Job1) at 100k. I brought it in at about 50 or 60K for this and a couple of other things, but the service rep. at the dealer said to wait until then 100k. I waited and there wasn't a bunch of difference in performance, so I theorize that the old ones weren't overly bad.
And, by all means, follow THIS procedure:
Day before changing plugs - go to store, get favorite adult beverage.
Day of changing plugs - drive to dealer, drop-off car for plug change, get ride home, enjoy adult beverages, pickup car fom dealer, drive home, enjoy another adult beverage (with hands intact).
It's not worth doing yourself.