It's not scientific, but what we have found is it doesn't matter when you have
DVC put a specific request on your reservation - it's just a notation on the reservation for your preferences. You can do it at 11 months or 11 days, it probably doesn't matter. BUT, if you put in a request through DVC, when you do an online check-in for the room, do not select anything in the room request category. It seems that making a room request during online check-in overwrites what you had DVC put on the reservation - and the online choices are very slim and very specific. From our experience (and others might be different), we add a request through DVC as soon as we make the reservation. Then when we arrive at the resort, if that request was important, we go to the front desk to see if the request has been honored. If the room isn't ready for you yet, the cast member won't tell you the room number, but they can see where your assigned room is located. If the location isn't to your liking, they sometimes can make a room change for you. Not always, but they will try to make you happy.