I thought it was midnight GMT; I typically see the upgrade list show up for flights around 7 or 8pm my time on the east coast, depending on whether or not we're in daylight savings at the time. So anywhere between 48 and 62 hours ahead of the flight's departure time for flights originating in my time zone.

The beta waitlist display has been flaky for me recently; it has been there for several flights, but others just say "Upgrade Waitlisted" or "CPU Eligible" (for flights I haven't thrown PP at) even inside the window when I can view the waitlist via flight status.