46 hours ... if you're lucky! :) 23 hours for a command to reach the spacecraft and 23 hours more for the spacecraft's response to reach Earth. If you're lucky: the Voyager project has to compete with other projects for antenna time on the Deep Space Network. If they can't get two slots 46 hours apart, they rely on delayed telemetry to verify that a command was received and successfully processed.
Afaik we always have at least one downlink dish pointed at voyager all the time. At least, last i saw the DSN site
70-meter dish antennas are needed to transmit to Voyager and there is one 70-m dish at each of the 3 DSN stations in California, Spain, and Australia. The Australian station is the only one that can see Voyager 2 and because of the Earth's rotation, that's only for part of the day. Downlink can make use of smaller arrayed antennas (including non-DSN antennas), but I still think they have to be scheduled; i.e., the antennas have to be pointed at the Voyager spacecraft and computer time for ground system DSN processing of downlink data has to be allocated. I don't know for sure though, so you may be right.