Originally Posted by FallenPlat

Effectiveness of Global Entry should effectively be immediate, assuming you get approved a few minutes after the interview as would be the SOP. This stuff is all computerized, after all. The fact that the actual physical card comes a couple of weeks later and needs to be activated is completely irrelevant. The card isn't needed in this circumstance.

The harder part, though, is attaching a known traveler number to the reservation -- full correct names match, everything -- the day before a flight. I'm not sure exactly how many hours of advance notice the system needs for the KTN to work on an already existing reservation. (Once you're up and running, the KTN is attached to your airline frequent flier profile (something the you have to do manually, by the way) and then auto-populates all your new reservations going forward.)

Maybe someone else knows the answer to that question?

(Edit: Note that both relevant tasks are manual, and you have to do them step-by-step. That is, first, you have to manually add your KTN to all existing reservations, and, second, you have to manually add your KTN to your frequent flier profile so that you can get Pre Check for new reservations in the future. Note also that full names, birth dates, etc. have to match exactly.)