Battles aren't long enough to exhaust a trained soldiers, so stamina is un realistic at such small scales.
Except perhaps for sprinting.
Edit: Just did more scientific tests with my sword, 1h, swung it 12 times in a row then called it a quit, I could have went on and on, but you suggested 4-8 times before being tired.
Now keep in mind: 1) I am a lazy fuckhead in terrible physical shape and 2) Soldiers and Knights back then were trained and in good shape, as such they could swing their swords for far longer than me.
I didn't suggested that player should get tired after 4-8 swings, but to start getting tired faster after that spam if he continues to spam more. I agree that 4-8 attacks is too less, but mainly I suggest that considering spam attacking, I think guy with 2h could get tired a bit after let's say 10 swings one by one, and more swinging would tire him even more...
In the game generally everything going faster than in real life, so stamina should to.
And unfortunately there's no real sprinting in game, it could be nice to add to.
I don't want that players should carefully watch their stamina all the time, just when they go all berserk, they should get tired, and less useful for some time.
btw did you swing with your sword just like you want to kill some armoured guy? And while wearing your armor?