In general terms, the “Rule of Three” applies to the human body’s requirements for food, water and oxygen
- three weeks without food
- three days without water
- three minutes without oxygen
After three minutes without oxygen, brain cells start dying off - a process that is irreversible (and while that may sound humerous, that is all that is funny about it). Death is then likely within four to six minutes afterwards.
There have been some unusual cases in which people have survived at lot longer without oxygen, but that is exactly what they are - unusual.
The industry I work in requires regular retraining in CPR / Live Switchboard Rescue. Over the nearly 50 years that I have been engaged in this field of work, it has always been emphasised that seconds count. The longer you delay starting CPR, the likelihood of death increases exponentially.