Robert Comsol wrote:
Strange it hasn't been mentioned, yet. We would have to know the speed of a subspace radio signal for which we got figures in "Balance of Terror" and "The Enterprise Incident". Kirk's real time conversation with his superior in "The Alternative Factor" provides some clues:
BARSTOW [on viewscreen]: Negative. I'm evacuating all Starfleet units and personnel within a hundred parsecs of your position. It's going to be tough on you and the Enterprise, but that's the job you've drawn. You're on your own.
Assuming Barstow was not evacuating himself, this would indicate a real-time conversation (including video) via subspace at a distance of at least 326 light years.
In addition, we would need to know what is the maximum distance for "real-time communication". We've seen in TOS that there are times that the Enterprise needs to be close enough to establish real-time comm vs sending a message that they have to wait for a reply. (Example, in "Charlie X" the Antares was "barely in range" to communicate with the Enterprise when it was destroyed.)
For the TOS Enterprise, 100 parsecs could be traveled in a third of a day.
Robert Comsol wrote:
Balance of Terror
Janice's "finally" isn't clear. Either it suggests that it's really quite some distance (conversations with Kirk's superiors light years away usually occur in real time!) or it took Starfleet a while to discuss what orders to relay to Kirk.
The Enterprise Incident
KIRK: You understand that Starfleet Command has been advised of the situation?
TAL [on viewscreen]: The subspace message will take three weeks to reach Starfleet [and three more to get a reply]. The decision is yours, Captain. One hour.
Now that really takes some time. I'd find it hard to believe that subspace technology had gotten worse.
However, Tal's information could be wrong and Starfleet somehow made the Romulans believe that it would take that long. Kirk doesn't comment on the time a reply would take.
It's pretty likely that "The Enterprise Incident" takes place in an area of space much further away than in "Balance of Terror" and "The Deadly Years".