Ah! this is not the same question! So how much Oxygen is stored, depends on size of bottle, ( normally 76 or 114 Cu Ft) not all NGs have the same fit, only the Engineers and Boeing will know your fit, which can be found from a/c serial number and the FTM.
Boeing quote a dispatch pressure in PSI not volume as clearly, a larger vessel will have a different pressure to a smaller one, but all fits allow for the min requirement under FAA and ICAO Annex 6 requirements for flight above certain altitudes. Annex 6 refers
ICAO regs state that the mass flow rate must be capable of supplying sufficient oxygen at differrent cabin altitudes to meet the crew requirements, approx 1.4 LPM Secondary, 2.4/3.5 LPM Normal and 4.0 LPM High Demand.
To covert the size of the bottle to a time, multiply cu ft x 28.31 to get litres, then divide by flow rate, on 100% it should give you at least 30 mins.
Refs: ICAO Annex 6 section 4.3.8, 4.4.5 and 6.7
JAR OPS1 ( 1.3.8.5)
ANO sectio 45 and 54
Hope this helps.