they're air cooled, run them as long as you like. as long as you're moving they'll never overheat
Overheating is one thing, catastrophic failure is another. Chinagirls aren't really designed for all-day operation under heavy load.
They are primitive low-compression engines with always-questionable build quality that seem to be happiest below 6K RPM.
As soon as yet another motorcyclist-in-denial starts hopping them up, raising the compression and other mods, the chances for engine failure greatly increase.
Early "instructions" that came with chinagirl kits used to state "do not run for more than 30 minutes at a time".
(That may have been a blessing in disguise because many of those older kits would rattle your teeth out within 30 minutes!)
If one is looking for a reliable engine that can run all day long under load, the first place to look is at industrial/commercial engines.
Engines made by companies like Tanaka, Robin-Subaru, Honda and clones like Lifan and Huasheng - these engines will typically run as long as you want them to.