How long will a radioactive isotope decay?

Until there's none of it left. Different radioactive isotopes decay at different rates. And a bigger pile of it will last longer than a smaller pile. Without knowing WHICH radioactive isotope you're asking about, and how much of it you started out with, the answer could turn out to be anywhere between 1 millionth of a second and 1 million years.

i'm on my way to history not a mistory.