The fuel source is what actually sets the limit

A standby generator connected to a natural gas utility line isn't limited by an on-site fuel supply the way a portable unit is — as long as gas service to the home continues, the unit can keep running. This is the main reason natural gas is the more common choice where utility service reaches the home: multi-day outages don't require refueling or monitoring a tank level.

A propane-fueled unit is limited by the size of the on-site tank. A larger tank (500–1,000 gallons is common for a whole-house installation) extends how long the unit can run before needing a refill, but it's still a finite supply, unlike a natural gas connection.

Manufacturer maintenance intervals still apply during long runs

Residential standby generators are designed to handle extended, multi-day operation during an outage — that's the point of the equipment. Manufacturers do specify scheduled maintenance intervals (oil changes, filter checks) tied to accumulated run hours, which is why the owner's manual for the specific model is the correct source for exact service intervals rather than a generic number that would vary by unit and load.

Load and ambient conditions affect real-world runtime expectations

Running closer to a unit's rated capacity generates more heat and puts more strain on the engine than running at a lighter load — this is part of why correct sizing (not oversizing or undersizing) matters for long-term reliability, not just for whether everything turns on.