At least a day is right. Don't forget to sign up for your ride out to the presidential hangar. There's some quite interesting stuff out there (in addition to the presidential planes).
One nice difference between the Museum of the USAF and the Smithsonian is they let you in some of the planes. You can actually stand on the point where President Johnson took the oath of office in the plane that was serving as AF-1 at the time.

This museum, the Smithsonian Air and Space (both downtown and the Udvar-Hazy Center) and the Pima Air and Space Museum/Amarc (Davis-Monthan)/TItan Missile museums are must see for any aviation buf.