[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.