snackyx,
Marriott will book 50 weeks out. Their website is very efficient in helping you see what is open... put in your dates at the property you want and then scroll down a bit to the AAA,AARP etc. area and check "Marriott rewards redemption".
I have found there were many Caribbean destinations open for this summer as late as a week or two ago.
If you choose not to or cannot book for the full length of stay indicated on your voucher you can still use it but will forfeit the remaining nights. Likewise you can pay for additional nights if you wish to add on.
Also, it is not necessary to have the voucher in hand when making the reservation, nor is it necessary to even have all of the points in your account at the time of reservation. You only need to order the voucher about a month before you intend to go, but make sure at time of res they are aware you are using the voucher to pay.
Also, rumblings that a change in the larger award packages is imminent, so if you do have the points available you may want to prepare to act quickly.
Hawaii properties, especially Maui and Kauai, do seem to be tight and best to reserve 50 weeks out. Some properties like the new JW Hotel in Cancun are very easy to book right now.
Enjoy!