Speaking of SOP, it is also standard to graduate somebody back to breathing on their own regardless of how quickly they recover. I had what was supposed to be outpatient surgery a couple months ago. When I came out my oxygen level was too low to dismiss me so they hooked me up to an oxygen blast and admitted me. Within a couple hours I could take the mask off (when the nurses and my daughter weren't looking) and my oxygen was fine but they kept me there for four days because that is SOP.
At least that's what they told me. Might have been that they just liked watching me walk around in my butt gown.