There are historians who say that it is an exaggeration of the chroniclers. Most medieval battles were skirmishes with a few hundred soldiers. What is known from modern recreations is that a soldier in full armor cannot remain more than two or three minutes in direct confrontation before falling exhausted. And an archer cannot keep firing more than 20 minutes straight. If we take that reference, a whole day of fighting would have to be quoted. Perhaps half a day they were positioning the armies, and the other half collecting the wounded, and a little while of direct confrontation. Or there could be multiple attacks and withdrawals: a soldier goes to the front to confront, and when he is exhausted he returns to the rear to gather forces to continue fighting. These things are what the specialists say, I do not say it.
You are correct, deploying troops would take up a few hours more than likely, and the troops on the front line would absolutely be replaced throughout the battle by rear ranks, either as they became exhausted and wounded or as more men died. You also have troops like archers and cavalry who might spend half the battle basically waiting around until an opportunity for a charge presented itself, or until their missile fire could be effective. All of those things are still part of a battle though, and that's why they could easily take all day.




