The box rating typically shows output that the humidifier is CAPABLE of producing in a 24-hour period. The amount of time the unit will run without a refill (run time) depends on the combination of the output and the capacity of the unit. For example: A 4 gallon output humidifier with a 2 gallon tank capacity will run about 12 hours on high before running out of water. The speed the unit is being operated at and the amount of moisture the room needs will also effect run time. This is why the run time is stated as "up to" a certain number of hours.