A 68-year-old man with severe pneumonia and septic shock is being transferred from the Emergency Department resuscitation room to CT after appropriate measures have been taken to stabilise him for transfer. He is receiving oxygen via a non-rebreathe reservoir mask at 15 L/min. A full portable CD oxygen cylinder with an integral valve, pressure gauge, regulator and oxygen outlet is attached to the trolley. Continuous monitoring is planned during transfer.

Assuming continuous oxygen delivery at the current flow rate, approximately how long will this full CD cylinder last before it is empty?