In a closed nuclear fuel cycle, most 240Pu will be fissioned (possibly after more than one neutron capture) before it decays, but 240Pu discarded as nuclear waste will decay over thousands of years. As  has a shorter half life than  the grade of any sample of plutonium mostly composed of those two isotopes will slowly increase while the total amount of plutonium in the sample will slowly decrease over centuries and millennia.
In a sealed nuclear fuel cycle, most 240Pu is fissioned (possibly after multiple neutron captures) before it breaks down, whereas 240Pu abandoned as radioactive waste takes thousands of years to decay. As a result, the proportion of  in any plutonium sample primarily made up of those two isotopes will gradually rise while the overall amount of plutonium in the sample will gradually decrease over extended periods.