This is the opportunity 'of a lifetime'. = This sort of opportunity comes along 'once in a lifetime'. [=this is a very rare opportunity]
a baseball player's 'lifetime' batting average [=batting average measured over a baseball player's full career]
She was presented with a 'lifetime' achievement award. [=an award for all the work she has done in her life]
(..) and whereabouts, you'll have your visa. And the honor of having served the Third Reich. I was in a German concentration camp for a year. That's honor enough for a lifetime. You'll give us the names? If I didn't give them to you in a concentration camp... ...where you had more persuasive (..)1942 Casablanca