From National Geographic

Medical science ia always learning more about how much a body can take.  Yet as Duke University physician Claude Piantadosi notes, "At some point it's impossible to rescue yourself."  Here's current thinking on the extremes of human endurance. 

Most of the following numbers are estimates for a 150-pound male with no special gear.

Body Heat                       107.6F
When core body temperature hits 107.6F, heatstroke can't be reversed and will prove fatal

Cold Water                      40F
Water saps body heat.  You'd last barely 30 minutes in a 40F sea.  Life vests buoy you up to slow heat loss.

Hot Air                            300F
In a burning building of deep mine, adults can take 10 minutes at 300F.  Kids soon succumb in a 120F car.

High Attitude                   15,000 feet
Consciousness fades for most.  With bigger lungs and more red blood cells, highland dwellers are okay.

Diving Deep                    282 feet
Without equipment, most folks black out before 2 minutes and below 60 feet.  The best free diver made it to 282 feet.

Lack of Oxygen              11 minutes
Typically, you'd pass out within 2 minutes.  With training, people can hold their breath nearly 11 minutes.

Blood Loss                     40%
You can survive after spelling 30 percent.  At 40 percent, you'd need an immediate transfusion.

Starvation                       45 days
Lose 30 percent of body weight and death is imminent, though disease will likely kill you before you starve.

Dehydration                   7 days
Every cell needs water.  Replace the quart or so you lose daily, or you won't last much more than a week.