Everytime you renew your subscription you can choose to renew it for your additional retainers too. If you don't, they'll still be there but cannot be accessed, any item stored within them will not be accessible and they will stop selling their items. You can re-subscribe for them (or even only one of them) at any time, and everything will be accessible again. I don't know if they expire after a set amount of time as I've only have had them unsubscribed for few hours