Currently, you can pay in full anytime, but cannot book anything (excursions, adult dining, etc) until your booking window opens. If you go online to check into your reservation to pay, you'll see the exact date you can start booking. Varies depending on how many times you've cruised with them before.
You are not charged for anything you book online, such as excursions, until the activity happens. So your onboard account will be charged during your cruise as things happen. After applying your onboard credit, if any, and other things such as prepayments (for example, some people prepay gratuities), Disney gift cards you take to guest services, or cash payments you take to guest services...your onboard account will be billed to your credit card at the end of the cruise.
Long answer, sorry!