Even if you are able to tolerate lower levels of oxygen you won't survive if the debreather mask stays on the entire time since the device will eventually absorb all exhaled CO2 which leaves only Nitrogen at 78% and other trace gases each at levels under 1%. The human body can't store oxygen, once it runs out and is not replace we eventually die. The only available oxygen should be from what is in your lungs and your blood.
From what I've found online, oxygen levels under 55% will result in loss of consciousness and then death, however there are other factors and variables which may result in the process taking longer than expected. I am planning on buying a pulse oximeter so I can monitor how rapidly my oxygen levels drop.
My biggest concern with death by hypoxia using the debreather is my survival instinct kicking in as I start to lose consciousness. I will probably take a sedative and anti-anxiety medication to reduce my SI. It's critical that you aren't disturbed after becoming unconscious since cellular death with begin throughout the body, with the brain being the most critical as it consumes 20% of oxygen in the body. If the process is interrupted it could result in brain damage at varying levels.
Hope that helps!