I've wondered that myself, since some locations on maps are sometimes so far out that there shouldn't be any way our intrepid heroes could make it there (Galactic Barrier, anyone?) but I think Joss Wheldon said it best concerning ship-speeds in 'Firefly' and 'Serenity' - a starship travels at the speed of plot.
Exactly. ENT and TOS are the worst at being inconsistent with distances, but then TOS was never meant to create a whole, continuous universe that fans like us would be analyzing with the equivalent of an electron microscope years later. In TOS, the Enterprise regularly travels the radius of the galaxy no problem. ENT even placed Qo'nos within a few light years of Earth if you look at travel time in just the first episode. But yeah, it's definitely determined by the writers and what plot they want to use, figures be damned.
The plot drive is far faster than warp, slipstream, or transwarp