you can extend a wall 16 times, because there are 17 places to put a wall on the board. Of course you can only cast 2 walls per action(extend can be used once per action), so it would take at least 8 actions on top of the original wall-casting action.
The wall will be made of different materials, since there arent enough vines, earth, stones, steel, fog, fire, or bones to make a wall that big. If you are asking why it doesn't count as the 17th extension when a wall is destroyed and replaced, to me that is repairing a wall rather than extending it.
I hope I answered your original question.