No one knows. I had situations where apparently a flight had still loads of seats and when unexpectedly I needed to fly and award flights were dead cheap last minute - and of course the revierse, that last minute booking would be expensive. You only can check. As a general rule, if you see a good miles offer for award flights on dates that suit you, take it. Even if months in advance, chances that you will get lower prices closer to flight dates are very uncertain.