Which one is correct?
1. My flight was tiring because it was a twelve-hour flight.
2. My flight was tiring because it was a twelve-hours flight.

Question

Which one is correct? 
 1. My flight was tiring because it was a twelve-hour flight. 
 2. My flight was tiring because it was a twelve-hours flight.

Answer

Number one is correct.  'A twelve hour flight' is an adjective, adjectives are never plural.

Answer

Good English would be:
My flight was tiring because it was twelve hours long. OR
My twelve-hour flight was tiring. (It’s obvious that its length would make it tiring)

Answer

12-hour 🕣