I have racked up over 110k miles total on two e90 M3s. It's my opinion that plugs should be changed between 20k and 30k miles. I've noticed the difference in running at 20k miles. The dealer was inconsistent with me as far as when they were changed. Once told me they weren't due; then next time I came in they told me plugs were overdue. I seem to remember mid thirties as the mileage the dealer changed them at. I change them myself at roughly 20k intervals.
Not good for your coils to let them go too long before replacement.
Dinan compliment of stuff plus PF rotors and RG63s. Enough for now.
Why, yes. I am an abrasive bastard.