I replaced mine at 32k miles which was about the 3-year mark. I had a really hard time taking three out of the four plugs out as the design is such that ~3/8 inch of the plug thread is exposed inside the combustion chamber and mine had a little carbon build up on the threads making it hard to unscrew. A lot of back and forth to loosen it up and not damage the head threads.Mileage interval is 75,000 Km. That's 46,602 miles. I don't put many miles on my Metris. 3 years was nowhere close to 75,000 Km. Time doesn't affect a device like a spark plug as it would affect oil. I'll replace the plugs strictly on a mileage basis.