On average you should be changing the platinum plugs at about 80-100k's for maximum fuel economy and performance.. at 120k, the car will still be fine but idle a little rough.
Then again it is all dependent on the type of fuel used and the conditions the car has been put through,
e.g starting the car and switching it off straight away all the time is really bad for platinum / iridium plugs, so is octane booster!
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