My experience is VERY limited and anecdotal. However, I've found that super-low domestic mileage awards appear starting around 10 days before the flight, and vary a LOT between then and flight time...I'm talking about meaningful "price" changes multiple times a day. I've seen strange things like I "purchase" an award ticket, and then immediately run the same search and "prices" have gone DOWN (so I "refund and repurchase"). YMMV, of course.