Answer:
An ML6 oxygen cylinder, when "charged" to normal pressures, holds 170 liters of oxygen. How long that will last depends on a number of factors.
If the flow is set to 2 liters per minute continuous then it will last less than 1 and 1/2 hours. If an oxygen conserving device is used (they give the gas in pluses when breaths are taken) it can last over 4 hours.