We did it a bit differently when I was training in the Canadian miltary. We had the masks in place with the hose disconnected from the oxygen and a plug to keep the line open. We did a similar type of test, with much the same results you indicated. Once we became non-responsive one of the chamber technicians would connect our house to the oxygen again and it was amazing how quickly we recovered. We got to watch others go through it so that we could also recognize when someone (a co-pilot) was becoming hypoxic.
One thing we did learn was that each person has a different set of symptoms that they can identify before becoming hypoxic. Mine are a tunneling of vision and a distinct feeling of heat in my forehead. I still recall this after some twenty-five years. Unfortunately, not everyone that could be put in a situation where hypoxia is a possibility has the chance to take this type of training. It is very insidious.