Wiktionary
once-in-a-lifetime
a. That is likely to happen only once in one's lifetime.
Usage examples of "once-in-a-lifetime".
A huge dominecker rooster, seeing a once-in-a-lifetime chance, dashed in and pecked him severely about the loins.
She accepted a once-in-a-lifetime job offer and went away, and now she was married to some super-terranean that took her on cruises through the rings of Saturn and was wealthy beyond belief.
Maris had always been content to wait until that once-in-a-lifetime love happened to her, too, rather than waste time by flinging herself without thought into a brief affair with any man who just happened to have the right physical chemistry with her.