H size Cylinder holds 6900 liters @ 10 lpm =690 min or 11.5 hrs
@ 3 lpm = aprox 33 hrs
Know the lpm of your device & do the math
That really depends on how fast you use the oxygen, if you fully open the valve to air the cylinder will vent off (empty) within a couple minutes, if you just use a little at a time it could last for hours, days or even weeks.