Typically, the amount of time you can run your generator continuously is determined by the size of the fuel tank and the efficiency of the engine. You should never fuel up the generator while it’s running, so you’ll need to turn it off when the fuel runs out. But, if you’re using a continuous supply of propane or natural gas, heat build-up is the main concern. Depending on how well ventilated your generator is, you may be able to run it for up to 24 hours at a time.