The general consensus is T-5/3/1, then "rounded up" to 5pm eastern time the day before.

Lately there have been a lot of posts about few/no upgrades clearing before the gate, despite FC being wide open. Not sure when they zero out upgrade inventory in those situations, but it's at least T-6 or before, and seems to be somewhat random. To/from hubs and elite-heavy routes seem to be especially subject to gate-only upgrades.