It didn't take very long, and I have my settings to put about 600ish units on the map at a time. Overall the first battle eliminated 1700 of them and took probably ten minutes. The second half of the army is still alive though, I should approached it from the side so I could collect all the enemies instead of half of them.
If you have a high leadership skill and keep your moral high (easy to do if you don't have many units) it's pretty easy to maintain your party. You can also turn it off (since it doesn't really make sense anyway).