Six Months: If the blood test comes saying you cat has the correct level of immunity against rabies, you must wait six months from the time the blood sample was taken.

Blood test now and you will get a form in the post saying Annie is free of rabies.
You will wait 6 months and get her retested and you will get another clear form. She can fly the next day as long as the second test is clear.

The reason for the wait is if a cat had been infected just before the first test the first blood test might come back negative, but the cats second test 6 months later would show the rabies.