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]
(..) you ordered the Code Red on Santiago? - Listen, we're all a little-- - I'm sorry. Your time's run out. What do we have for the losers? Well, for our defendants... it's a lifetime at exotic Fort Leavenworth. And for Defense Counsel Kaffee... it's a court-martial! Yes, Johnny, (..)1992 A Few Good Men