If you hold your breath, your lungs will explode as the gas expands - unless you want embolisms, exhale! Some suggest yelling but thats probably over the top for the flight deck. Given that this is the case, then you are exposed to low O2 partial pressures almost immediately.
Another thing (relevance unknown) is that the body uses blood CO2 levels to determine when to inhale - does this have an effect on time of useful consciousness as the CO2 is washed out?