A large D-size portable oxygen tank lasts five hours at full capacity with a flow rate of 1 liter per minute. Full capacity for this size tank is 2,000 pounds per square inch. A larger E-size tank with the same pressure and flow rate takes 10 hours to empty.

Oxygen tanks can last between 0.75 and 37.5 hours. How long they last depends on the size of the tank, the pressure in the tank and the flow of oxygen, which ranges from 0.5 to 5 liters per minute. For example, a D-size tank that is full last only 1.25 hours at 4 liters per minute but can last 10 hours at 0.5 liters per minute. Some oxygen tanks have conserving devices that limit breaths to 20 per minute, which allows them to last longer.