To be accurate, the Voyager
was quoted with "stable
cruise velocity" of that numerical value by Lt. Stadi in "Caretaker", and "top
cruising speed" of that numerical value by Capt. Janeway in "Relativity".
While both expressions emphasize cruising rather than dashing, thus suggesting the speed can be maintained for a not insignificant period of time, the word "sustainable" was apparently fumbled out of existence by Ms. Coppola when delivering the "Caretaker" line...
We never learn how long the ship could have cruised at this speed. In the show itself, she is repeatedly shot to hell and no doubt unable to meet the design specs, as demonstrated e.g. in "Threshold" where we learn that trying to match warp 9.9 prompts the computer to warn about "imminent structural collapse" and, soon thereafter, "structural failure in 45 seconds". We never learn what maximum speed the ship reaches during that very brief chase, but Chakotay feels the chase can safely continue if they throttle back to warp 9.5. This would appear to be the "top cruising speed" or "stable cruise velocity" of the vessel during that episode, then.
Not bad at all, considering that warp 9.3 was a "redline" for the Galaxy