New cards every three months. Once a month is too often to have to update every current deck to acknowledge new possibilities in opponents' hands.
Balance updates shouldn't be on a set schedule. It seems to me that this would cause cards to be "balanced" arbitrarily, especially if balancing seems to come in the form of adjusting costs of the most/least played cards without consideration as to why they are that way. On the other hand, if the balance changes are due to changes in the overall card pool, then aligning them to the new card schedule makes perfect sense.
Balancing changes that correct behavior that could be considered as exploitative should continue to come immediately, of course.