I can give you my experience. I planned to use frequent flier miles for a Southern Caribbean cruise on RCI using Delta miles. I figured I better book pretty far in advance. I called in September for flights at the end of May, and already there were no ff coach seats left at 30,000 miles per person. However, for 60,000 miles each, there were 2 first class seats available, which I took instead of double mile coach tickets which would have cost the same. It was the first time I had ever redeemed ff miles, so I happened to have 120,000 miles for the two tickets.
I would think to Alaska departure ports the seats will go very quickly, since for a lot of the country that is a usually a relatively expensive flight.