What happens if you die from anemia how long would it take?

Fast or slow: It all depends (as with everything!) Ultimately one would die from lack of oxygen transport to the tissues of the body. If this comes in the form of a heart attack, it could be fast with a relatively-high hemoglobin (perhaps even 8 if you had severe heart disease). On the other hand, if anemia declines slowly over long time in young person, I have seen Hb of 3.