Actually, if you check the
timeline it was about 10 minutes to be exact.
What can happen in 10 minutes?
“An F-15 departing from Otis can reach New York City in 10 to 12 minutes, according to an Otis spokeswoman."
F-16s could have also reached DC within 10 minutes, but that never happened:
"NORAD orders three F-16 fighters scrambled from Langley Air Force Base in Virginia to intercept Flight 77. Langley is 129 miles from Washington. Ready aircraft at Andrews Air Force Base, 15 miles away, are not scrambled."
Why did this not happen? Because there was a constant 30-45 minute delay in the information being relay from the FAA to NORAD.
However,
the President was getting information in real time via CNN. There are fighter squadrons at the ready throughout the country that he could have scrambled on a moment's notice.