At the gate, lately!

The auto sweep is at 5pm eastern time (takes a few minutes to run) and the notification emails come within 2 hours after the upgrade is processed. It will be your upgrade window, rounded up to the day before.

If it doesn't clear at that time, it is done manually and could be anytime from then until the departure, though usually at the gate if not at the window, as Delta is trying to sell those F seats until the last second lately.