The weird thing is that two of the times the Enterprise saucer was separated, it explicitly made or was intended to make a long interstellar journey. In "Encounter at Farpoint" it beat the battle section to Farpoint despite having started off going in the opposite direction at Warp 9, and while the saucer is supposed to be able to sustain warp speeds separating at warp was treated as being unusual and it was still a considerable distance. In "Arsenal of Freedom" the saucer is dispatched to a starbase which has to be several light-years distant (at a minimum, and it should be much further given no one's noticed that planet before) without even starting off at warp. The other two times it either is unclear what happened ("Best of Both Worlds", where it got away from the Borg cube under its own power) or it promptly crashed into a planet, rendering the FTL question moot.
Personally, I would assume that the saucer has its own warp engines, possibly powered off its fusion generators. While it would obviously be much slower than the battle section unless given a starting boost, it could still be enough to get away. The warp engines might be those two big sets of glowing blue panels. I know some people think they're botanical gardens, but then there would be no reason for them to glow, and having big windows would be bad for the plants anyway. On the other hand, Federation warp engines always glow blue.