Ships in the Star Trek universe can't seem to sustain speeds greater than Warp 6 for extended periods of time without suffering major engine damage. Regardless of what its top sustainable speed is (which may be only sustainable for 12 hours as far as we know), most Federation starships seem to routinely hover around Warp 6 for really long trips and only use their maximum warp speed to cover a relatively short distance very quickly, IMO.
A relatively short distance, of course, depends on the imagination of the writer. In some stories it could be simply to the nearest system, in others it could be a sector, or the distance between Earth and the Neutral Zone...