How long can the human brain survive without oxygen?

About 6 minutes before brain cells start to die. If a person has a cardiac arrest resuscitation has to begin within this time to be successful. The time may be prolonged if the body is cooled, for example by drowning in ice cold water.

As mentioned, temperature (specifically metabolism) plays a very big role in how long you can go without oxygen, but as general guideline:

• Heart: Needs constant oxygen
• Brain: 4-6 min
• Kidneys: +- 30 min
• Muscles: +- 2-4 hours
• GI Tract: +- 12 hours

Thus, the main areas of concern is the heart and the brain.
A human body ca only live for six minutes without oxygen, before the brain cells start to die.