In this text I present several cases of survival in the death zone on Everest without oxygen. People have been shifting limits and boundaries all the time.
It is surprising to realize what people can survive. There is no definite answer to the question on how long you can survive without oxygen on Everest.
But there are some particular cases that show that people can survive more than 20 hours without oxygen bottles, and this far in the death zone on Everest.
Incredible Lincoln Hall story
There is a New York Times article about Lincoln Hall case. In short, after reaching the summit of Mount Everest, Hall was struck by cerebral edema, a severe form of high altitude sickness. This caused hallucinations and he turned and tried to climb the summit again.
The Sherpas tied him and moved down the mountain, but after two hours he was showing no sign of life. After discussing by radio with the leader in the camp, they were told to leave him and to save themselves. His family was informed by phone about his tragic end.
Some 12 hours later, in an early morning a group of ascending climbers found him sitting up with his jacket unzipped, frostbitten, weak, but alive.
He did not have gloves, hat, oxygen bottles, sleeping bag, and this was at sunrise at 28200 feet (8600 m). So this was only 249 meters of elevation difference below the summit and well in what is called the death zone. He was in the stage of paradoxical undressing.
Now, to these 12 hours in such conditions, one should add 19 hours from his ascend and descend to the point where they left him.
He died 6 years later at the age of 56 but of something else. He lived to tell the story, you will see him in this fascinating video, please have a look:
The Babu Sherpa case
Babu Sherpa has spent 21 hours on the summit of Everest without oxygen tanks, and survived.
He was on the summit 10 times before that. His name is the Guinness World Records list for the fastest ascent from the Base Camp.
Reinhold Messner case
According to the same Guinness World Records source, Reinhold Messner was the first to successfully climb Mt Everest solo in 1980. It took him three days from his base camp at 6500 m (21325 ft).
He did not use bottled oxygen. It is easy to understand that on his way up he was in the death zone for quite a number of hours, not clear how many really. In any case, he is still well and alive.

In summary, people can survive much more than anybody can expect. This is all individual and there is no way to know in advance who can survive longer.
Did you know that they were examining Messner to see if he had anything special that would explain his achievements? Could not find anything. It seems it was more in his mind than in the body.
However, in the case of Sherpas, there can be some biological reasons that make them such extraordinary mountaineers. There was an article about this several years ago.
Thank you for reading. For more texts of this type please check under the category Mountaineering here in the site. Let me know if you have questions or comments, there is a comment box below.