My recent experiences are of seeing little or no F Avios availability more than two or three weeks ahead. I have been successful in booking Avios seats in J and calling up if and when any F space opens up. It has required flexibility however - in September 2 seats to Male opened up at 4 days notice and I was lucky that we could move our departure forward a couple of days. A return trip to Miami this month opened up in F but only around 3 weeks before each individual flight and only single seats.
I agree that availability is poor and only really usable if you can be flexible (and / or you can regard it as a bonus on a J booking).