Frizze is right - SE doesn't bill your card til your subscription time is up, depending on the billing cycle you select. Added into this is when you first create your account and set up your subscription, SE counts the first 30 days as a 'free' period, which after that ends when your card is first billed, the time from then on is counted towards the Veteran Rewards if paying by credit card or debit card.

Because of this, if you don't want to wait for the items it's probably better to track down a time card and use that for your subscription, as it's the only way to get your Veteran Rewards up front (to a certain point - there are restrictions to how much time you can add to your account, up to 180 days if I remember rightly).