There is no hard and fast rule to this unfortunately. While on occasions additional F seats can open up closer to departure, with the reduction in F seats across a lot of BA's fleet (14 down to 8) and the desire for people to pay for a bit more comfort at the moment, F seems to be doing reasonably well in terms of being filled on a lot of flights, avoiding the need to release a seat. BA revenue mgt also seem to be managing Airport Upgrade Prices better with the desire to squeeze a bit more cash out of the impulsive traveller at check-in vs. releasing the seat via Avios. It obviously will depend on the day of travel / overall flight load etc. so there is no firm answer I can give you I'm afraid, but for an honest one, I would be surprised at more then the current 1 seat opening up in F unless the overall flight load is still quite low,

Pilot37