Shoot till they are down. I've seen to many well hit hogs make the trees.
No matter how far away that the trees are!
I have played that game, shoot a hog well, knowing it was a good shot, and watched him disappear in to the distance. You may shoot a hog well, and still have a bullet that fails to do the damage that you want/need for the hog to drop quickly. Do you know if a hog is going to run 20 yards, 100, 200, 400, or half a mile? What if that well placed shot actually managed to miss the vitals? It happens.
It is the same sort of mentality mistake made by a lot of people engaging in gun battles. They think that because the person they shot in the chest is going to die, they stop shooting, only to have that person kill them.
Shoot until the threat is eliminated.