How long would it take for plutonium to decay to one eighth of its original activity?

For plutonium (or any other radionuclide) to decay to one eighth of its original activity, it will take 3 half-lives of the material. In one half-life, half is gone. Half will be left. In another half-life, half of the half that was left is gone, and one quarter will be left. In a third half-life, half the one quarter will be left, and that's one eighth of the original. In the case of plutonium, there are a number of isotopes of this highly radioactive stuff. The isotope 239Pu, which is commonly used in nuclear weapons, has a half-life of 2.41 x 104 years. That's 24,100 years. For 239Pu to decay to 1/8 th of its original amount, it will take 3 time the half-life, which is 7.23 x 104 years, or 72,300 years. And yes, that is a long time. A very long time....