What's heard exactly twice in a lifetime?

LONGI

Think of a Latin-derived word for "long" that appears in a common phrase about life's fleeting moments.

The clue plays on the phrase "long I," which sounds like "long eye" but is actually the letter "I" stretched out. In a person’s lifetime, the word "long" is said once, and the pronoun "I" is said countless times—but the sound of a long "I" (like in "time") only appears twice: in the words "I" itself and "eye." The answer, "LONGI," combines "long" + "I" to hint at that drawn-out vowel sound.