A massive heart attack is considered to be a more severe heart attack than a regular heart attack but it doesn't always mean that you won't live a good long life after the massive heart attack.
However when you do suffer from a massive heart attack and do live afterwards then if you don't change your lifestyle and get more exercise and eat more healthy then you might only live 5 or 15 years after the massive heart attack.
But if you do change your lifestyle and get more exercise and eat healthy then you could live a regular lifespan after the heart attack.
Your current age also factors into the lifespan after a massive heart attack as if you're 80 years old and have a massive heart attack and survive then you might live another 5 or 10 years.
If you're 50 years old and have a massive heart attack and survive then you may live till 80 or 90 years old.
There's no way to know for sure how long you'll actually live after a massive heart attack but sometimes a massive heart attack will shorten your life and other times it will not.