I think it might be helpful to your worries to think about what has to happen for you to NOT get what you want assuming you get nights 1-7 already.
(1) All the other rooms in the category you want have to book up for Night 7 as well.
(2) All those other bookings also need to want Night 8, all of them.
(3) The inventory for your booking category needs to be reduced for some reason. In other words the number of rooms available for Night 8 needs to be at least one less than what is available for Night 7.
The chances of both (2) and (3) happening are quite small.
If you are super paranoid about it, call every morning for Nights 8-12.
If you recognize that the chances of not getting 8-12 are basically nil, call 5 days after you booked 1-7 and add on the rest of the stay.
What you could do is look at the RAT and see if Night 7 is available. If it is, then you know with reasonably high certainty that Night 8 is not in danger of getting booked any time soon (To book Night 8, the owner must also book Night 7). If you see Night 7 is booked, then go ahead and call MS to add as many days as possible, then check the RAT to see if your last booked day sells out and repeat as necessary until you get what you want.
Edited to add: OOOPS!
Oh Sorry. I completely messed that up. I missed that you were talking about booking at 7 months. In that case, calling everyday is the only thing you can do. Since 7 months can get dicey, I would consider a split stay as well, especially with a long stay.