Do note that from 1 turn and above the AI will have the same ability as you in that it takes them just as long to recruit the unit as it takes you to recruit that unit. However if you make it take 0 turns to recruit a unit it will only be 0 turns for you. This doesnt mean you get the unit in the same turn but when you queue up 6 units you get all of them in the next turn, here is where the AI wont be able to keep up with you as they never 'queue' units as you can, they just recruit 1 unit per turn (at most).

So "imo" you should never go below 1 turn recruitment time for units so the AI wont be lagging behind in recruited and you can just wals over them.