Every 32,000 Km or so. Suggested by a automotive engineer. That would be roughly 20,000 - 25,000 miles. I've seen motors taken apart as low as 36,000 miles and the intake valves had severe carbon deposits causing poor running issues. But again it's to each their own. I would like to keep my vehicle for at least 10yrs this time. I'm retired and I'm not going to replace my vehicle every 3yrs like I use to.