Voyager 1 and 2 are the most distant man made objects. How long will we be able to still communicate with them? What will be the first thing that prevents communication? RTG exhaustion? Signal attenuation over the vast distance? Something else?
Popular Mechanics had a neat article about it last year. Bottom line, nobody knows. We will lose communication with the probes at some point in the next 10-15 years because the fuel supply will run out. The probes are powered by nuclear reactors and scientists expect them to be depleted sometime in the 2020's. They have been proactively shutting down systems in order to conserve fuel and extend the life of the probes, but there is no hard and fast date they expect to lose communications.
Source: Voyager 1&2