If the upgrade is going to process when the window opens, it usually processes within an hour of the window opening (never precisely at the time the window opens, because I check). More and more though it seems there is little rhyme or reason as to when or why they process. I got an upgrade not too long ago at the window where they filled the F cabin to all but 1 seat at T-120. For my upcoming flight in 3 days, no upgrades have processed and 12/12 seats are available in F