It used to be processed in the late evenings/early mornings but these days, it can be anytime.

For a redeye last Thursday, the upgrade came late Friday, for 2 flights the following Friday, 1 flight was UG'd before 2pm PT, the other still unUGd. Of the return flights Monday, the 1st was UG'd before 2pm PT and the other 2 not UG'd yet. Don't know when these flights will be UG'd if ever so it pays to keep checking (but P seats won't be available until checkin.).

The unupgraded flights have 3/16 or 4-5/22 allocated in FC at most.

The 2nd flight on the return leg just got UG'd after I posted this.

The 3rd flight following the above got UG'd about 1 1/4 hrs after the 2nd.