I tried to get an accurate time but with variables like
1. amount of targets in range.
2. AI chooses a time limit for every battle. Based on units.
3. and If you choose no time limit.
its seems like 3 mins on constant attacking especially archers even if they still have 40% ammo left that are tired and shooting slows to a crawl.
and swordsman/pikemen/spearman get tired in about the same time frame and they will just stand there and get slaughtered.
Now you could argue attacking in waves but when the games AI charges 1000 men at you waves arent an option...
I hope DoF Unit AI is better I really hate it when a 500 of my troops are sent to attack and maybe only the front 50 are really attacking I would assume the Total war series couldnt work out particle collisions cause if the units are not of the same type they dont stand side by side with others and fill in the gaps