Tough to say, because the harder you run your car the shorter your glow plug life will be. And R/C cars are generally run harder than airplanes. And, yes, when the temperature and humidity change you will have to tweek your needle valve some more. Safest thing to do (especially if you don't have a temp gun) is before you first start your car up for the day open the needle valve a 1/4 turn from where you last set it. After you get it running carefully turn it in a click or two at a time and do an acceleration check. When your car accelerates cleanly without bogging from a standing start you've got it right where you want it. Clean but not too lean.