Once you request your reward card, the card is good for two years or so. After you receive it, you can continue to load additional reward dollars on it.
Disney Visa site says 4-6 weeks to receive your card but I don't think I have ever waited more than 1 week. You can request it now and then continue to load earned reward dollars onto that card until your vacation.
When you use up all your dollars remember to keep the card! It's not like a gift card where you throw them out once you have used up the money.
We love our Disney Visa...I put everything on that card. We have $1000 to apply to our December trip! I love knowing that every time I spend something a little bit of money is being set aside for our vacation.