Do we know how long the power source will last?
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/
Also:
Even if science data won't likely be collected after 2025, engineering data could continue to be returned for several more years. The two Voyager spacecraft could remain in the range of the Deep Space Network through about 2036, depending on how much power the spacecraft still have to transmit a signal back to Earth.
That FAQ covers a lot of interesting ground (though it talks about 2020 in the future tense).
After Voyager 1 took its last image (the "Solar System Family Portrait" in 1990), the cameras were turned off to save power and memory ...
I didn't realize that was the last image.
... it is very dark where the Voyagers are now. While you could still see some brighter stars and some of the planets with the cameras, you can actually see these stars and planets better with amateur telescopes on Earth.
Since it’s powered by an RTG, how does the power get “used up”? I assume that this refers to the available power budget at a given moment versus some sort of expendable power reserve.
It would be quite depressing if it was us failing to receive a fainter signal, rather than Voyager failing to send it.
One of the longest-running scientific experiments, too. It's already about as old as the Queensland pitch drop experiment was when Voyager I was launched.
Supposed to run out in the 2020s