Memory Alpha mentions there to be a definite upper limit as to the speed of Subspace Communications. Is the reason for this ever touched on in any of the Star Trek media (Canon or not) or is it "speed of plot"? This question would suggest speed of plot but I'm curious what would provide an upper limit if light doesn't?
Using the period specified by Memory Alpha, the speed of subspace communications is estimated at Warp 9.999 by the 24th century. (Not quite the speed of plot, but definitely within the distance to allow a mission directive and an independent captain to make a decision without a being able to call for an update. He or she would be on their own...)
The only period I have metrics for the actual speed of a subspace communication is the 24th century when a communication between galaxies (the Milky Way and Andromeda) at 2,700,000 light years would take 51 years and ten months.
Using that metric, subspace communication speeds, assuming no degradation would be 52,123 times the speed of light. A speed so fast no Federation vessel could even come close to matching or sustaining for very long; the equivalent of warp 9.999.
But even at that speed, the distance between Earth and Alpha Centuari (a distance of 4 light years) would have a lag time of ten minutes! Perhaps in tightly populated areas the system of relaying would maintain signal speed only allowing the signal to become slower in areas without subspace relays. Sending a signal from one side of the galaxy (100,000 light years) to the other would take approximately 1.92 years.
Check out the graphic below:
Assuming a map with reliable coordinates, subspace relays and origin points, one could tell how fast each period's subspace comms traveled. It is safe to assume the technology grew progressively faster every century. Judging from the graph, subspace communications during the NX-01 would have been Warp 8 or so. During Kirk's time, Warp 9.6 might have been the average (far faster than the ships but still taking a four days to a week for most communications within 20 light years.)
It is also safe to assume, baring any reasonable disruption, ion storm, magnetic disturbance, solar flare, black hole or subspace anomaly, communications in any reasonably populated area of the Alpha quadrant can be considered near-real time by the 24th century. Only the farthest points in the Alpha Quadrant or communications with other Quadrants of the galaxy would have any significant delay.