Here are the supplemental oxygen requirements from the FAA:

Sea Level to 12,500' - No oxygen required.
12,501' to 14,000' - Required by the required crew if over 30 minutes at this altitude.
14,001' to 15,000' - Required to be provided and used by the required flight crew.
15,001' to 25,000' - Must be provided for every occupant.

Since the Duke has a pressurized cabin, the oxygen system is only really designed for use in a pressurization emergency, or to supplement the cabin pressurization at very high altitudes, as you discovered. There are also different types of oxygen masks. Some are more sophisticated "diluter-demand" or "pressure-demand", which use less oxygen than the standard face mask or nose canula, which are "continuous flow" devices. In fact, continuous flow devices are limited to only 25,000ft by the FAA, so you should not rely on their use over that altitude, but they could still be useful in an emergency scenario.

In reality, many continuous flow systems have adjustable flow rates to prolong cruising times, while my system is just on or off. This is probably the biggest contributor to what you're seeing, since the body would not require the maximum flow rate to sustain a healthy blood oxygen concentration at only 12,000, once acclimated.

All this to say, you should be able to avoid all oxygen concerns by maintaining within the limits the FAA imposes. However, if your blood oxygen concentration begins to decline below 90% on the tablet, a period of supplemental oxygen use, followed by continued monitoring of your blood oxygen concentration should keep you in good health to fly without using all of the continuous flow oxygen in a few hours.

I hope that helps answer some of your questions! Let me know if you have any more.