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]
(..) Okay, two if you count the barbecue apron with the bikini body on it. I don't blame you, Jay. You'd had enough excitement in one lifetime-- The dawn of flight, printed books, agriculture. All right, what's with all the jokes? It's insult comedy. I've been watching old (..)2012 TvShow: Modern Family Title: Bringing Up Baby Season: 4 Episode 1