My dad has a 13 KW essential load generator. It ran 4 days and nights during Ike.

The 3600 rpm air cooled generators are not built to run constantly for days at a time.
They need to be shut down and let cool off now and then. It is a good idea to run a
synthetic oil in them also. Helps keep temps down and oil last longer. Mine suggests
change oil every 2 years or 200 hours.

The water cooled 1800 rpm generators can run longer without shutting down due to
turning slower. But they cost almost triple and are bigger in size and a lot heavier.