Not always. Lots of episodes they are well out of range of communications with Starfleet. They have even mentioned not getting a response from Starfleet for weeks in a number of episodes.
Except in Voyager (where due to the nature of the premise they had to be _somewhat_ consistent) this was entirely plot-driven. It could be anything from "real-time comms halfway across the galaxy" to "we'll get a reply next week" to "no contact at all". Occasionally this even varied within the same episode.
