Your heart is one of the most vital organs in your body other than your brain and liver and kidneys.
Your heart is the pump for your blood that helps to carry oxygen through your body and to your brain and when you get shot in the heart it causes your heart to loose blood and likely will stop the heart from pumping depending on where your heart was shot.
If you get shot in the heart and don't get medical treatment you could die within minutes or could lay there and die after 20 to 30 minutes.
Once you get shot in the heart you'll usually go unconscious within seconds because of shock and loss of blood.
It takes your brain 30 to 180 seconds for your brain to stop functioning correctly without oxygen so you would loose consciousness within that time.
So taking a shot to the heart is not always deadly but can be within minutes if you don't get treatment and get surgery on the heart to fix it and remove the bullet.