Once in a lifetime is a phrase that means an event or opportunity that is not likely to be repeated. The phrase once in a lifetime was first recorded in 1854, and can mean an event or opportunity that literally will not be repeated within one’s lifetime, or more usually, is an exaggeration that refers to an event or opportunity that happens very infrequently.
If the phrase once in a lifetime is used to describe when the event or opportunity happens, (it happens once in a lifetime), it is used as an adverb and is not hyphenated.
If the phrase is used to describe the type of event or opportunity it is, (it is a once-in-a-lifetime event), then the phrase is being used as an adjective and should be hyphenated as once-in-a-lifetime.
For fans like Craig Davis seeing his favorite player off the field is something that happens once in a lifetime. (The Tyler Morning Telegraph)
“Just take the time to really enjoy it because it only happens once in a lifetime,” he said. (The Sun Sentinel)
This is a story of a special love that happens once in a lifetime — if you are lucky. (The St. Augustine Record)
About their impending wedding, Rohit says, “A wedding happens once in a lifetime and, to find a life-partner of your choice is like a blessing. (Hindustan Times)
But she needs help to fund her once-in-a-lifetime trip and is calling for the people of North Wales to support her. (Daily Post)
The companies offer clients once-in-a-lifetime entertainment and learning experiences — for example, a special trip to the Olympics or VIP service for corporate groups traveling to big events. (The New York Post)
Dallas Arboretum’s 20-year-old Victoria Agave plant begins once-in-a-lifetime bloom (The Dallas Morning News)
Its website still advertised the Magnificent Misty Fjords by Floatplane tour this morning, describing it as a “once in a lifetime trip” to see “towering granite cliffs, 1,000-foot waterfalls, lush and remote valleys and serene crystalline lakes”. (The Independent)