Pretty good most of the year from what I've seen, right at 7 months. Even if you can't book right at 11 months, booking at 8 months or even 7.5 months would be usually wide open at Poly. 7 months exactly is when owners at all the other resorts can book Poly.
If you really think you'd rarely be booking at 7+ months, you might consider a resort with cheaper points, since all points are the same at 7 months.
Also as you may or may not be aware, they are currently building an additional
DVC hotel as part of the Polynesian resort. If you buy Poly now, your points may also be good at the new resort when it opens, and they may not. Disney hasn't said one way or the other, and it's been a source of much speculation and discussion around here.