[Izkata]

Several episodes mention subspace relay stations, and between how communication seems to jump between real-time and waiting hours or more for a reply (I don't recall any minutes-long delays), I have a suspicion the unwritten rule is subspace communication has some sort of speed decay. Like it actually slows down the further it has to go, instead of going at the same speed but weaker, and subspace relay stations intercept the message and re-send it at top speed. More advanced technology in the later-set series could also then just send it faster than in the prequel series, explaining how they communicate over larger distances even though it's supposed to both be subspace communication.

[smogcutter]

I think the unwritten rule is that subspace communication takes precisely as long as the plot requires it to. (Especially in the more modern series)

[hef19898]

Star Trek is an extreme case of the plot driving the tech, and not the other way round. They manage to pull it off, most of the time, using ingenious Starfleet engineering and technobabble. For some reason, with Star Trek it works most of the time.