Aircrew perform a 5 second check on their mask before each flight and the system itself is bled for 15 seconds at the start of each flight as part of the pressure check. So there's about 30 seconds of 100% oxygen bled off per flight.
730 psi represents about 40% of the system's capacity and, by my calculations, is 4-5 times more than what you'd get from the pre-flight and start up checks.