once in a lifetime
Extremely rare, especially as an opportunity. For example, An offer like that will come just once in a lifetime. This phrase, often used hyperbolically, was first recorded in 1854. Also see of one's life.
Examples from the Web for once in a lifetime
The current energy and industrial boom, according to Siemens President Joe Kaeser, “is a once-in-a-lifetime moment.”
For people who love history, this is a once-in-a-lifetime, once-in-history resource.
Not that this once-in-a-lifetime spectacle is likely to come to pass.Bring on the Made-for-Reality-TV District Race Down South|Michelle Cottle|February 25, 2014|DAILY BEAST
Investors will view the Facebook IPO as a once-in-a-lifetime event.Beware the Facebook Hype: Investors Encourage Company’s Insane IPO|Dan Lyons|February 1, 2012|DAILY BEAST
Charity auction site charitybuzz to host a star-studded line-up of once-in-a-lifetime experiences.