Well, I suppose it depends on what sort of 'magi' you have. Warlocks sacrafice their own blood and self to cast their magic, so the more powerful the warlock, the longer it will take for him/her to tire. Magi use their own energy to cast their spells, which draw upon both the Well of Eternity and their own knowledge of magic. The more powerful the mage, the longer it will take for him/her to tire. I may have been a tad bit misleading with my previous post, but my point still stands. Also, @The Madgod, I know an army of magi would be almost impossible to acquire considering their rarity. It was an example I put forth, not an actual scenario that could stand a chance at occurring.
Warriors will train to become accustomed and used to the weight of their armor/weapons, but will last longer because the energy they use to fight is their own. A more conditioned, experienced warrior won't waste all his energy on pointless attacks, but use it wisely. So, again, it all depends on who you have fighting for you.
And Roggles . . . sorry, but I'm not going to look for hours on end on Google and WowWiki to try and find the quote you've requested. I doubt one even exists because of the facts I've just put forth. ^_^