Radioactive materials have a half-life, which means every X number of days/months/years, the material will have decayed to half its initial size. This continually happens, so while something might start at say 100g and have a 10 day half life, 10 days later it is 50g, 10 days later 25g, 10 days later 12.5g, 10 days later 6.25g, etc. This will continue forever until there is literally one atom left, at which point it is random chance when the atom would decay.

Voyager launched with 3 Radioisotope Thermoelectric Generators, each with 4.5kg of Pu238 which generated 157 watts of usable electric power at launch, for a total of 471 watts

It will be half that in 87.7 years (235W) . And half that (117W) in another 87.7.

The problem comes from the amount of power necessary to run the craft and it's radio. While the generators might continue to run for practically forever, at some point they just don't produce enough power to keep everything working. Its official mission is expected to end in 2025, but it's generators are expected to be able to power it's instruments for another ~10 years