YMMV. (IOW, it depends.)

I stubbornly waited until nearly 37K mi on my original plugs. I'd noticed

the car had less part-throttle torque and put in Bosch Platinums. Big

improvement. I've replaced the BPs at ~ 30K since then. Tiny improvement.

The BPs seem to stay new-like much longer, apparently because the special

center electrode is protected from the heating/rounding that inevitably

causes conventional plugs' firing voltages to rise as they age. My car

came with "conventional" plugs.

Their "+4" (multi-electrode) plugs are not approved for turbocharged

engines.

I've also stubbornly (car failed to start a time or two), I mean,

scientifically tested my distributor cap/rotor to failure. 100K mi is the

outer limit. The 'contacts' were very pitted, and so much metal had been

transferred that plenty had been thrown in the circle the allowed arcing

to begin to become a problem apparently (img of cap follows on web site):

- Dave; '95 854T, 101K mi