I think it also depends on the destination, and whether the destination is in the "high season". I had a flight during the summer from HNL to DFW to EWR. The DFW to EWR was upgraded 6 days before the flight (at the time there were no booked seats in FC), but the HNL to DFW wasn't upgraded until the day before, even though there were 9 FC seats available. Even at 4 hours before the flight, there were still 2 unclaimed FC seats. I am not sure the algorithm used by the yield management software, but you can be sure it is programmed on the stingy side.