The plutonium 238 decays according to a curve, and the thermocouples are degraded as well by heat and radiation according to a curve. So the power output drops rather predictably: "The radioisotope thermoelectric generator on each spacecraft puts out 4 watts less each year." [1]

The Voyagers will soon no longer have enough power to operate any of their instruments. They'll have enough power to continue operating the transmitter (which serves as a science experiment of its own) into the 2030s. The power of the signal will drop before the electronics and control brown out (if it works as designed), and it the signal might become too weak to detect before the probe completely stops operating. Such a fate befell Pioneer 11, who may yet still be warbling away at low power no longer pointed at Earth; its carrier was last detected in 2003.

[1] https://voyager.jpl.nasa.gov/frequently-asked-questions/