Welcome to the forums! I haven't tried time card game sub + credit card retainer before, but I believe they will follow the usual formula of pro-rated remaining active sub * monthly cost. Since you're paying your game sub via time card, my guess is that your retainer most probably wouldn't be on a recurring payment and will simply be "freeze" once it timed out until the next manual payment. Just don't freak out when you can't summon your retainer in-game when it's payment run out, all data will still remain, you simply can't access it until you make the next payment.