It also depends on how many of the battle you wish to manage with your dragon. I once took on a battle that I had 0% chance to win. With the dragon I won it, but it took me an hour and a half to do it. By far the longest battle I had and I never did it again.
So if you just auto resolve, using cards and generals, it can go fairly quickly. If you use your dragon every single turn, it can take a lot longer.
