Years ago, in the military, we were in a pressure chamber at a simulated 25000 feet. I took off my oxygen mask and found I thought was just fine till the Sgt told me to put the mask back on.
I felt OK, but everyone in the chamber said I turned a beet red from lack of oxygen. I’m going to say, you can not last more than a minute or two.
Also, when depressurization comes, the cabin fills with fog.
And before you black out, you go retarded. They gave us simple puzzles and toys to operate in our altitude chamber orientation. A short time off of the O2 mask, the puzzles became impossible.