We just got back from a Beach Cottage, we booked it 11 months out(day by day) for our June stay.
For info only, I tried calling at the the 7 month window for any week in June and MS told me NO Beach Cottages were available for the whole month of June except 1 or 2 days only on a friday or saturday.
I would try to switch pts with a VB owner and book at the 11 month window if you don't want to take any chances.