If you plan to pay with points, you can only book a crusie 11 months prior to departure.
I have read that you cannot book a future cruise for points while on board. That said, I belive I read something to the effect that some have been able to "reserve space at a special points price" when there was a special promotion going on. IIRC, you got some type of certificate and were to call MS when you disembarked. Still couldn't book until the 11 month mark, but the "price" was locked in. I may not remember exactly, so do confirm this with someone else and ask when you are on your cruise.
I don't think you will have a problem getting a cat 5 or 6 at the 11 month mark, but you may not get the exact cabin you want. Those who book for cash do not have to wait until 11 months to book. They also have more favorable cancellation/refund policies.
If you are set on a particular cruise, you may want to consider booking it for cash now. Then you could rent your
DVC points to someone else and use the proceeds to pay for the cruise. This will require some work on your part and it may not be worth it to you.
I think it would be worth it for me to book for cash - if something happened and we couldn't go, I know I could cancel (within the deadline) and get my money back. (You would get your DVC points back, but there would be restrictions on using them for soemthing else and you would forfeit the $75 booking fee). If I did cash, I wouldn't have to pay DVC $75 booking fee and if I wanted to make a change to my reservation, I could without paying another $75 fee. I could also get a better choice of cabin (within the category) because I would be able to make my reservation sooner than 11 months prior to departure.
Whatever you decide - have fun!
Best wishes -