My day ended around 3:30 - 4:00. So if you're catching a flight right after your interview, you'll have to face rush hour traffic in Atlanta. The earliest you should schedule your flight for is probably 6:30, and later is better.
If you're going back to the Atlanta Int'l airport, you can take this bus from across the med school building to the MARTA train station. If you take the MARTA to the airport, I think it takes about half an hour.