It was partly addressed in Enterprise with them dropping buoys along the way, but how fast do you realistically think communications should be at the time of Enterprise? We know there's FTL travel, but how fast would FTL communications be at that point? Surely not so that 90 light years out, they have instantaneous communications with Earth, even though that was shown a few times on screen.
It's subspace radio, which is always as fast or slow as they need it to be, just like how long it takes to get somewhere at Warp X before they must do something highly important.
I seem to remember one episode they made a point of dropping a satellite or something. Which was destroyed. Anyways, distance never stopped Forrest from calling up Archer and having a live chat with him.
I imagine that communication within range of these relay satellites is virtually instantaneous, as seen in Enterprise and recently in Into Darkness (where Kirk phones Scotty on Earth from the Klingon border). But when communication between parts of this satellite network is disrupted (which could mean anything from some space anomaly to damage to malfunction) or a ship is out of range, the signals have to be sent the slower, non-boosted way which is what we see in all the episodes and movies where communication takes days or weeks to reach HQ. In other words, long-distance subspace communication from a ship to a distant target is slow, but when connected to the satellite network it's virtually instant.
Which makes perfect sense when you look at it the way Okuda and Sternback did in the TNG Tech Manual. Their brilliant idea was that messages travel in subspace at extremely high speeds - but only for a short distance. So a message sent by a ship outside the reach of a buoy network does not travel more slowly as such, it just travels fast until a certain point, and then crawls to the nearest relay buoy. Different ship transmitters would have different range, translating to different delays in messaging. And with small craft, it could be that sending a message to Starfleet might take aeons, but receiving one from Starfleet might be quick, because even the relay buoys have longer-ranged transceivers! This would account for certain DS9 anomalies with runabout communications, for example. Timo Saloniemi