You may have heard that DC's Metro system has seen better days. In fact, Metro is a total disaster and its about to get worse. Much worse.
Starting June 4, there will be single tracking on various lines for days at a time. In August, if they are on schedule, the single tracking will be:
Red Line, Shady Grove to Twinbrook, August 9-18
Blue and Yellow Lines, Franconia-Springfield to Van Dorn, August 20-Sept 5
Single tracking means that trains can use only one of two tracks so they have to stop the trains going in one direction to let trains in the other direction go by. It causes delays - usually extensive delays - along the entire line.
If you are arriving at Ronald Reagan National, that's Blue Line so you should be OK on arrival but if you are leaving on Saturday the 19th or Sunday the 20th, leave at least two hours to get to the airport. Add another hour or two for the security lines at the airport! (Actually, the TSA lines at National haven't been that bad - generally no more than 30 minutes).
Better alternatives - taxi or Uber. Taxis may be hard to find and Uber may be charging higher rates, but the airport is not far so fares are not very high. Share a cab if you need to save money.
To take Uber, you need to sign up in advance: https://get.uber.com/