The prolonge in the French service of the period was used to both advance and withdraw. It was attached to the trail of the piece and the other end to the limber.
It was from 38 to 42 feet in length, and 27 feet were 'left' after it was tied to the limber and the trail of the piece.
Further, it was not a drag rope. That was another piece of artillery equipment that was employed to move the piece by its crew on foot. The prolonge was meant to be used with the gun team and limber for movement.
The Civil War prolonge in the US service was 26 feet 7 inches long and was used for both advance and withdrawal, 'and for various other purposes.' It was stored on the trail of the field piece. Again, it was essentially the same piece of equipment and used in the same way as its Napoleonic ancestor. See The Artillerists' Manual by John Gibbon, page 297. There is also an illustration of it on a plate between pages 296-297.
For the prolonge in the French service ca 1809 see Appendix III of Artillery of the Napoleonic Wars, page 266-269, with the illustration of the implement on page 268.