Well, I'm pretty sure that Dagorlad lasted over a long period of time, months probably. In Appendix B of the Lord of the Rings it is said that the battle took place in 3434, and along with crossing the Misty Mountains before the fight and beginning the Siege of Barad-Dûr afterwards in the same year, I bet there would've been at least a few months for the battle to take place. In Of the Rings of Power and the Third Age, in The Silmarillion, on p. 293, it is said that no greater army had been mustered since the War of Wrath. So they had plenty of soldiers to last a while, too.
Also, my Atlas of Middle-Earth, by Karen Wynn Fonstad, says:
Quote:
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The battle was waged on the stony plain of Dagorlad, north of the Black Gate of Mordor, "for days and months", and the bodies were buried in what became the Dead Marshes.
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I cannot find her own quotation from Tolkien, "for days and months", anywhere, but I trust her to accurately quote Tolkien.