Quote Originally Posted by Morindin
A man in combat would probably be able to fight for around 12-15 minutes (and that's a trained fit man, with some generosity. I'd probably tire after 5 minutes of straight combat, provided I didnt die in the process) before becoming exhausted. If there is no second line to pull back behind it doesnt matter how big your army is, you'd rout or die.
However, the other side would also tire at a similar rate. Fair enough to say that if one side has reserves and the other doesn't, the side with reserves will likely win, but I doubt that in battle it was wise to replace your front line as soon as the men there became tired. I think that combat took quite a while before reserves were employed, and short periods of fighting were interspersed with slightly longer periods of resting and heckling for both sides. It appears from reading accounts of battles that the side which commited its final reserve last had the advantage, so there would have been a certain skill in judging how late you could leave reinforcement. Too long and your army routs. Too short and your whole army tires.

Indeed, I believe the Romans, in one of their early battles against their fellow latins, concealed their triarii by having them kneel to trick their opponents into committing their final reserve, thinking the Romans had already used theirs. The Romans won that one.

So it's not simply a case of fighting for ten minutes and then being rotated. You'd have to fight for longer than that (provided as you say that you didn't die in the process).