how far is this airport from Pentagon

That is a tricky question with a lot of possible answers. Depending on the months of the year (school or summer vacation), days of the week, and the time of day, and the mode of transportation, you've got yourself a combination of possibilities. However, at the very best situation, if there was no traffic and you were still traveling at slightly above thr legal speed limits, you should be able to get to Dulles from The Pentagon within 30 to 45 minutes. However, I personally would plan for at least an hour or and hour and a half. Even then, that is kind of pushing things and assuming things will be smooth. Roll the dice, and prepare to play.

If you go into Google maps and enter the two way points and hit directions you will magically get your answer