Oxygen Endurance
In the US, FAR 121.333 requires enough oxygen for each flight crewmember to have 2 hours' worth: 10 minutes' worth during a constant descent from the max certificated altitude for that aircraft to 10,000 feet, and then 110 minutes' worth at 10,000. It doesn't address some of the possible variations of flow rate (e.g., 100% vs. normal, normal vs. emergency), nor does it address different requirements for cargo carriers who utilize Class E compartments.
As far as flow rates, 100% at one atmosphere (i.e., sea level) is very, very high, but decreases as cabin altitude goes up. Emergency flow is fixed regardless of altitude (on most masks) and normal flow (again, on a "typical" mask) results in almost no oxygen being used at lower cabin altitudes, but will increase oxygen in the mix until you're breathing 100% oxygen -- even in Normal -- at higher altitudes, typically between 18,000' and 25,000'.
Another way of putting it....If you need 100% oxygen and you're not sure your supply will last as long as you need it, consider climbing the cabin altitude. Of course, if others are on the aircraft, this won't always be practical....