Your missing the point. Three, six, ten, it makes no difference ~ it's the risk/reward ratio that matters. How many times can one raid in a season? That's the only way to know whether it's even feasible to have a set max that has nothing to do with the upkeep of the unit. 

EDIT: 

Also, how long are your turns supposed to be? I mean, really, your going to post every in game week saying, "yup, my soldiers are still pillaging the countryside,"? I thought the point of having seasons was that your posts each covered one season. I mean, I suppose you could order your units to just stay in an area until enemy soldiers came and made them leave, but even so how long can a group of soldiers stay in a territory pillaging before the enemy shows up with a host and kills you all?