I would guess it depends on the day and time of day. Last Wednesday I took the 10:00 pm ATL-SFO flight. Booked an L fare 5 days out. Was UG'g 3 days out and there were still plenty of F seats open after that. I would guess GMs got upgraded too. There were lots of open seats in Y and the only thing offered in F was the snack basket, so this was one time where it may have been better to miss the UG.