I just read a book about the ancient Rome Legion, the author has a pretty unique idea compared to the history books I read in other places:

ancient battles were mostly short burst of melee fight that lasted just a very short period of time rather than some ancient tales saying fighting "whole day" bla bla bla.

one side, often those leniently orgniazed, badly-trained troops, would collapse once those behind the first line thought the situation looked or sounded (they couldn't really see an exact picture of what was happening to the first line) and ran away. That was when the real slaughter began, as the chasing of the fleeing troops started.

Actually, won battle I do know of invovling the Romans that went for most of the Day, the Battle of Pydna.

The Macedonians continued stopping the Romans advances and pushing them back.
Unfortunatly for the Macedonians, their flanks (The Companions and the Hypapsists) didn't march forward as fast as the Phalanx, the the Romans out flannked them.

Well once the Phalanx was out flanked, the slaughter begun inside the Phalanx formation, while the Companions and Hypapsists tried in vain to help them.

Wether the Greeks could win or not truly depended on wether the people behind the first line collapse or stay.

First line will stay and fight if they have their comardes behind them standing firm (no way they can escape either), but if their comrades think Romans are flanking them and they flee, the battlefield will become a slaughterfield.

in ancient time communication was really backward and soldiers really didn't know what was truly happening, battle probably would just last for 10-20 min and one side would flee

Yes, the Greeks were like that, that's how the Macedonian Conqured them.

Never call a Macedonian a Greek and never call a Greek a Macedonian.

The Macedonian Phalanx was a very unique structure. First of all, evey last soldier in the Phalanx was a professional. They were not un organised militias trained to fight in war time. They were highy trained and disciplened professional soldiers trained to fight all the time.

Secondly, The Macedonian Phalanx was 48 ranks deep and x number wide. The first six ranks, held their sarissa (a 20 ft long spear) out infront of them, while the remaining ranks held their in the air.
If a soldier in the front rank was killed, the soldier behind would move up and tak his place, and this would continue throughout the formation.

This tactic worked so well, because the Phalnx would hold out long enough for the Companions to run in and cut the opposing flanks, while the Hypapsists would close in on the main enemy formation. Once the Enemy broke ranks, it would be a large scale butchering.

Another unit though that was unusual to Ancient tactics was the Theban Sacred Band. A 300 strong formation of soldiers that could fight, cut off from the main formations.
Their shields were designed to overlap the next persons shield, protecting them, and they would fight to the very last man.

They were completly destoyed by Alexander the Great's Charge at the Battle of Chaeronea in 338 BC, they were the second last Greek unit to be destroyed that day. The last was the Combined Greek Phalanx, that Alexander later surronded and attacked from the rear. Destroying the Bulk of the Greek armies, and letting Philip II to conquer Greece.

do you think they just push pikes (medival word, but pretty much the same thing) for whole day?

or it just lasts like less than one hour before one side's line will collapse and the battle ends with cavalry charge against fleeing enemies.

Well the new research into ancient warfare has shown that even humans in peak physical condition lose significant effectiveness after 5 minutes when trying to hack slash and thrust their hefty weapons while wearing all the armor. So new theories have come up to say that ancient battles where more 10 minutes of fighting followed by 10 minutes of resting back and forth kind of combat.

I'm pretty skeptical about this model but it is a new theory that is out there. One thing is pretty sure, it would be very near impossible for an ancient Roman/Macedonian/Greek infantryman to stay in the fight for an hour, much less the whole day.

some examinations of ancient soldiers' remains show that most of people were killed when they started to flee and enemies' cavalry started to chase them.

ancient battle tends to be a short burst of fighting (sort of like street fighting in my opinion, lasting no more than 10 min while everybody is exhausted) that ends with one side's quick collapse.

but i am not sure about the Macedonians' pushing of pikes with each other, probably lasting longer

You want to know what a ancient battle is like watch the first part of gladiator cut out the hollywood crap add double the men and double the carnage. picture bodies all over men on the ground screaming from being cut down, shot from arrows, smashed from heavy cavalry and thats a ancient battle. Like a lot of guys have said so far ya they are not that long battles few minutes 10-20 mins. the battles that last a "day" would not be a full out conflict most liking small skirmishes.

Macedonian pike was great in phalanx formation but as I see it there is huge draw backs. First off you have hundreds of men close together Archers?? slingers?? skirmishers?? Also the flanks like people said flank the pike they in trouble. The way I see it also if you can get face to face the men in the phalanx you have the advantage since they have pike and the romans have their short sword. I mean ok its built in layers but the rows behind dont have much room to move or anything.

What you said about people found most soldiers died while running away
darn right. I would cut the guys down as they run I mean its like we just fought a huge battle your not getting away that easy. I know the Romans had a cavalry unit that was made to do only that they traveled light and would kill retreating units.

I found a couple of pictures just from a game it gives a fairly good idea of a phalanx formation saidly I did not get one of a huge phalanx formation.

lol, another ROme total war fan? maybe we can play a couple games together sometimes.

Rome total war is by far the most accurate re-enaction of ancient warfare among all other games and movies made (this game is even used by History channel for the programs talking about ancient wars).

but I still have doubts about how is it possible that my legionaries running around and fighting for whole day without exhausting themselves to death.

in the game they do get very tired they will not fall over but they will move much slower and not fight as great. Ya though its a great game for kill ands history lovers.