I have it the other way around.
I attack an AI city and kill off most of their militia. Then, on the same round come in with my second force to finish the job and, low and behold, ALL the miltia are there again, full health!
IMO, miltia should last the round. But if they're defeated or killed, they should NOT respawn until end of turn. IMO, they SHOULD even have a 5 turn cooldown...just like the heroes. Only seems fair.
But I do agree, on the flip side, if they survive the first encounter, they SHOULD be around to fight in the second.