How long does it take after someone has a heart attack to know whether they will survive or not?

Varies. If you survive immediate sudden death, prognosis can be determined within 24 hours for uncomplicated heart attacks especially if you receive rapid, advanced cardiac care. For large heart attacks with major complications it can take days to weeks.
Survive Heart Attack. This is a complex question. Every individual that suffers a heart attack is different and survival depends on many factors. Your doctor is the best source of information conserning your survival, as he can evaluate all the factors concerning your disease.