once in a lifetime - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.