If all goes well the conditional approval takes a few days and you may be able to get an interview slot in a matter of days from that, but you do have to get to a center. GE becomes effective immediately. Pre-Check requires you to provide the Known Traveler number to the airline and may require a refresh of existing reservations.

GE saved us some time at ORD recently when an unplanned bunching of international arrivals happened. The resulting slow baggage delay undid some of the gain though!