While Time of Useful Consciousness at FL450 is quoted as 9-15 seconds
In the event of a sudden decompression it’s closer to 5 seconds. You can’t hold your breath and your lungs will work in the opposite sense and dump oxygen from your blood to atmosphere.
If you can get to a breathable altitude within 90 seconds you’re probably going to be OK. The problem is that if someone has stopped breathing or suffered cardiac arrest from hypoxia, then they’re unlikely to spontaneously recover going below 20,000ft.