Global Entry interview on arrival: How long does it actually take?

Hi everyone!
My conditional approval finally came through (after 9 months!!) and I have an international flight landing in IAD next week.

I want to do the “Enrollment on Arrival”. If my flight lands at 6pm, will the interview office still be open? And does it add like an hour to your exit time? My ride is picking me up and I don’t want to make them wait forever.