It's better to ask: * How long is the flight? * How long does the flight take? These are more natural and commonly used phrases. While the other options ("How long is the duration of the flight?" and "How long is the flight duration?") are grammatically correct, they sound more formal and less conversational.

If you want to use the word ‘duration’ which is more formal, you should say What is the duration of the flight? (‘How long’ is redundant because the word ‘duration’ suggests a length of time.) ‘How long is the flight?’ is natural but a bit ambiguous because it could be referring to its distance in a certain context. ‘How long does the flight last?’ eliminates that potential ambiguity.

All are correct but #3 is the winner because it uses the fewest words.