Most deaths from heart attacks occur within from the onset of symptoms.

2 hours

The other term for heart attack is myocardial infarction (MI) and these occurs when the blood flow that brings oxygen to the heart muscle is severely reduced or cut off completely due to blood clot or the heart muscle become narrow from a buildup of fat, cholesterol and other substances that together are called plaque. 

The heart muscle starts to die within 80-90 minutes after it stops getting blood supply, and this critical time is a window of opportunity that influences a patient's survival and quality of life following a heart attack. If no medical intervention is done within two hours from the onset of symptoms, death occurs.