I would think that a country change time of ~ 1 hour (60 minutes) would be a decent setup. That way you would not have people changing teams and upping to nail you when you get to your next target.
As Kratzer mentioned that happened to us the other night. We had a few missions rolling and taking bases along the southern coast and one of the mission members for the first two left and reupped on the Allies and "happened" to be in the next discussed target area that we flew into. The player also "happened" to bring along 3-5 other Allied fighters which totally wiped out a mixed bomber/fighter/troop flight quickly.
So don't take this as a whine. Just as an explanation of why I feel we should have a time limit on changing teams instantly.