I need an adjective, 9 letters long, means lasting a very short time

I would say that an adjective which is 9 letters long, and means "lasting a very short time" is ephemeral. There are many words that you can use to refer to the same things, because ephemeral has many synonyms, such as transient, momentary, temporary, all of which have 9 letters, so you can choose whichever word you like best.