In my '98 i replace the stock Champions every 25k miles or so. They are so cheap and so easy to do that may as well!! I like to also keep ck of the cyl/plug contitions that often too. However, i side-gap them before installing which could necessitate more frequent replacement. A good rule of thumb i have found is when there is at least .010" increased plug gap from wear with std plugs to be fairly ideal. At 25k there is not quite that on this, but does not matter. At 80k on this XJ the cap and rotor still look good. The wire replacement will be a matter of time in use i think, not mileage. Jury is out on that one so far-