The new National Museum of African American History and Culture, located next to the Washington Monument on the Mall, is a popular attraction and passes are still a bit hard to come by. Check for availability on nmaahc.si.edu or call 800-514-3849 between 9 am and 8 pm Monday through Saturday and noon to 8 pm on Sundays. The limit is 6 passes per request.
The timed passes, which will be used indefinitely, are to control crowds and entrance wait times. Once inside you can stay as long as you want.