It should be noted that 'medieval' isn't really a thing in relation to your question. The length of the period is one of the many things debated by historians, and at its very longest lasted from the fall of Western Rome in 530 to the discovery of America in 1492 (though more commonly, the fall of Eastern Rome in 1453), and as such, warfare varied wildly between this period, as well as location to location. In Ireland you had clans battling out across country, as well as 'viking' raiders and then the British. The nature of this of course vastly different from 500 to 1500, and will vary wildly to, say, the Eastern Romans, fighting Bulgarians and Serbs and Italians and then the Arabs for over a thousand years as well.

There are also two conflicting fields of thought on how exactly the battle was fought. One side claims that battles were fought over longer periods of time, with frequent breaks in between as both sides squared off and fell back, until one side either broke or retreated in a more orderly fashion. This means side's would effectively cycle their forces until there was none left to give or they had no more will to do so.

The other view is that medieval battles would normally be fairly decisive, and would be a grind until one side's morale was shattered and they broke or, once again, retreated. That units would be in combat for a short period after a longer period of skirmishing and preparing. In this view, armies would camp near each other, do some light skirmishing and test the waters, and then the actual battle would take place in a single day or so.

I'm also not particularly familiar with any history of soldier's breaking formation and fighting in small groups like in film. Battle's would often turn into formations of soldiers against others, and when one side broke they'd be hunted down, but for much of the medieval period, formation's ruled. The spear was the most common and effective weapon on the medieval battlefield, and is at its most powerful when paired with other men with spears.

This is of course the core of the answer, but there are so many pieces of a battle it might require a bit more specification in exactly what you were curious about.

To steal an excellent answer given a few years ago by /u/naorban, which is about the length of medieval battles specifically, but might also shine some idea into the nature of the battles:

I'm doing some research on it now to gather averages. Alot of time in the battle was spent chasing down the routing soldiers, and not quite as much time was spent during the engagement itself.

The Battle of Agincourt lasted about 3 hours, not including time chasing down troops, while the Battle of Hastings lasted 8-9, including finishing off the troops that still remained loyal even after Harold's death. The Battle of Tours consisted of the 2 armies of Charles and Abd Al Rahman camping by eachother for a period of 6 days, where short skirmishinng took place, and then fighting on the 7th, which took a single day and then ended when Charles sent a scouting party to loop around the army during battle and free prisoners/cause disarray at the camp, which as expected drew some Muslim cavalry back to camp to deal with them, which was mistaken by the other soldiers for a retreat, and trying to calm the chaos Abd Al Rahman was surrounded and killed, the Muslims all retreated back to camp and left the next morning, so that was 1 day of fighting and 6 days of not much else.

To mix it up I'll throw in some other battles involving more than just England. The battle of Dorylaeum was a battle where Crusader General Borhemund and his men had to hold their ground against Seljuk forces for 7 hours until reinforcements arrived and saved them. The battle of Hatten was one where Saladin followed the Army of King Guy of Jerusalem for 2 days, as Guy's army slowly dehydrated Saladin's army tormented them unril surrounding them and on July 4th, fighting broke out that had finished well by the end of the day.

There was also the battle of Towton the one guy mentioned up at the top which caused a big plagiarism thing which was listed as 11 hours of fighting and 24 hours of chasing routing troops.

So you'll find in the end that it really varies a lot between battles, More often than not moving troops takes a lot of time, and in many cases there's a lot of times the armies will move around for position, wait, or skirmish until they feel they have a good opportunity to strike the enemy, which can take days. The actual fighting itself isn't likely to last more than a day or so, because people would get tired of fighting by that point, and most of the army would be destroyed. On top of that if they're chasng down soldiers that can go on for a very long time, they may possibly even reassemble the army and march after them until they catch them.