Went here in August and had only about 1.5-2 hours to spend. I could have spent all day there. Some many great planes to see (SR-71, Enola Gay, Space Shuttle Discovery, etc). You can spend as much or as little time as you want from 1.5 hours to 4 hours especially if you read the placards about each exhibit. Worth a stop if you are in the Dulles area. Plus it's free!