I'm getting a unit every 16 turns from allied CS in the medieval era. I'm thinking the spawn rate increases slightly with eras like the other CS bonuses, possibly -1 turn every era. I'm currently playing a military CS heavy game so I'm gonna check it out. I got 3 CS allies which averages around 1 unit every 5-6 turns. Military CS get more useful as time goes on since it scales with technology and era. Late era units are quite expensive wise.