Level 5?

All wars last less than 2 and 1/2 months, because thats how long it takes for a level 1 ZERO to become a level 20 HERO, with constant fighting in a level appropriate enviroment; and an Army is a level appropriate challenge for a Level 20 Hero.

All that doesn't even matter, because the next day after a war is declared, divinations are prepared and cast, and all the heroes from one side teleport to all the heroes of the other side and nail all the heroes from the other side to a tree, then go on to headhunt the leadership of whichever side they want to.