RE: CRRC 40cc kit
Not long at all, I run them with 32:1 for about 30-45 minutes. After about 20 minutes you can hear the engine change pitch and start running a little "different". At this point I will adjust the carb and set the idle speed screw make sure the transition is good, then I shut it off let it cool and restart. Nothing should have changed, get it good and warmed up and shut it off do a couple of hot starts (1 flip at idle), if all is good (always has been) it's ready to fly. From then on I consider it broken in and will run it on 50:1 Stihl synthetic forever, check mixture settings with the 50:1 fuel mix. There is more wearing in to happen over the next couple of gallons of gas but it is of little import, rings have seated already and the engines will hold rpm's steady. Enjoy!
Jody