If it works like every other subscription MMO I can think of, which I have no reason to think it won't, then when the game goes live and you input your box code in your account to validate your retail game purchase (or whatever code you have dependant on how it was purchased) you will probably have to input some form of payment method at that point (credit / debit card, paypal or whatever they accept). You will then be billed at the end of the 30 days free gametime, the amount will be dependant on the subscription length you have chosen (1 month, 2 months, 3 months etc).