As an expert in the field of medical science, I can tell you that the determination of legal death can be complex and varies by jurisdiction. However, in many places, brain death is considered the definitive sign of legal death, rather than the stopping of the heart alone. Brain cells are particularly sensitive to a lack of oxygen, and irreversible damage can occur after a certain period without blood flow. The time it takes for the brain to suffer irreversible damage due to a lack of oxygen (anoxia) is generally considered to be around 4-6 minutes without blood flow. After about 10 minutes, the damage to the brain is likely to be so severe that the person is considered to be legally dead, even if the heart is still beating. It's important to note that the declaration of death is a clinical determination made by medical professionals based on specific criteria, which may include the absence of brain activity, circulation, and breathing. read more >>

If not, you typically don't. For this, a general rule of thumb is that brain cells begin to die after approximately 4-6 minutes of no blood-flow. After around 10 minutes, those cells will cease functioning, and be effectively dead.read more >>