I never had an interview go that late. If I recall, mine were all finished before 3:00pm at the latest. Depending on how late you get finished though, you may have to worry about traffic on the way to the airport. So keep that in mind.
I've had an interview last until 6, but that was with an optional extra tour, without that it ended at 5. I don't know the particulars of Georgetown.
It really varies by school, some end as early as 1pm, some end as late as 6pm. Usually the school has a website or has sent you an email that tells you a rough end time.
At my Georgetown interview, everyone was done by 3:30 at the latest. I think the invite says 4pm to be safe...Theres a room to store your luggage as well so you can bring that to the med school building and be on your way to the airport right after your interview...hope this helps and good luck.