To clarify, and I cannot recall my RAF decompression training now, but I think somewhere between 15 and 30 minutes of 'awake' at 20k without O2 and a heck of a lot longer alive, so no great rush in reality.
A heck of a lot longer is right.
Just for interests sake, all the passengers on Helios 522 were alive when the hit the ground and that had been hours at thirty something.