https://www.skybrary.aero/index.php/Time_of_Useful_Consciousness https://www.skybrary.aero/index.php/Time_of_Useful_Conscious... You have 30-60 seconds at 35k.

https://www.skybrary.aero/index.php/Time_of_Useful_Consciousness https://www.skybrary.aero/index.php/Time_of_Useful_Conscious... You have 30-60 seconds at 35k. At 18k you'll feel it but you'll be down long before it's serious. (Although if it was unpressurized for the climb I wonder how much of the time you used up that way.) Real world data--a guy punched at such altitudes and lost his oxygen in the process. Blacked out, came to on the way down. Amazing survival--he was supersonic at the time and punched without blowing the canopy.