When engine performance and/or mpg decreases or every 100,000 miles. That is just based on my opinion. I am sure Honda has a different one! Agree with answer 1- ebans777 on the P.M. also. ....Another thought, have 'your' repair shop run a FULL scan (movie) of your car while it is performing at top efficiency and perfofmance. They can make a printout of these results, you get a copy of the printout, they keep one on file. Later if there are any, inconspicuous, problems have this repeated and the two can be compared to find the trouble. Works well for us at the shop for our regular customers as it can be a time saver on diagnostics and prevent misdiagnoses. Time is very valuable!! Again it's just a thought.
"Tuneup" is really a term that has gone away with computer controls, the computer makes all fuel and ignition adjustments as needed, the important thing is maintenance inspections to keep fluids and filters in good condition, the spark plugs are also inspected for wear and typically replaced between 75 and 105,000 miles depending on the vehicle