Do you have 6 months from the time you book or the end of your vacation? Also, do you have to pay the deposit right away or is it considered part of your package and due in 6 months? Thanks for answering this question in advance. It's been bugging me for days and I didn't know where to go to get the answer!
You have 6 months from when the charges post to your card.

So, you'll have 6 months from when the deposit posts to pay that portion, and then you'll have 6 months from when the balance posts (45 days before your trip)
So, for example, last year I posted by deposit in April (27th) and I had until Oct. 24, 2007 to pay it without being charged interest. Then my balance on the package was due by Jan. 27, 2008. This was posted on July 27, 2007 (45 days from arrival date).
The deposit this year, and last (and maybe further back) was $200.
I think it's a great way to manage your money as long as you know you'll pay if off before the promotional due date. This will be clearly shown on your DV statement. I usually pay it a day, or so, before, online just to make sure.
One thing to know about since when it happened to me a few years back I didn't realize it at the time. If you use your card for other purchases and you return something and get a credit on your account, it will reduce your promotional balance. For example, I had a balance of say, $900 on the Disney portion of my card, I returned some items to Target (bought on the DV) for $50. My promotional balance was reduced to $850. When I called DV to inquire about this they said that this was they way they did it. Just something to be aware of if you use the card to purchase items that you might return.
HTH.
Let me know if you have any further questions.