“千载难逢”，意思是一千年里也难碰到一次（occurring only once in a thousand years）。通常形容机会极其难得，非常罕见。“千载难逢的机会”可以翻译为“chance of a lifetime”，表示“An extremely important and/or fortuitous opportunity, especially one that is not likely to ever present itself again”。
“千载难逢”也可以用英文俗语“once in a blue moon”来表示，当我们说“something happens once in a blue moon”时，表示“it happens only very rarely”。
例句：
曾经有一个千载难逢的机会在我面前，我却把它浪费掉了。
I had the chance of a lifetime, and I blew it.
这样的机会对你来说真是千载难逢。
It’s only once in a blue moon that you get an opportunity like that.