From the A-Z guide on the Nats' site:
Hours of Operation (March - September)
Game Days: Monday through Saturday, 10 a.m. until 30 minutes after the end of the game.
Sunday, 9 a.m. until 30 minutes after the end of the game.
For all games with a start time of 2 p.m. or earlier, the box office opens at 9 a.m. and closes the later of either 30 minutes after the end of the game or 5 p.m.
Non-Game Days: Monday through Friday, 10 a.m. - 5 p.m.
Saturday 10 a.m. - 3 p.m., closed non-game Sundays & Holidays.