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]
(..) Bye, Dad. Bye, Dad. Bye, son. Barbara. l don't understand it. Here this thing has a lifetime guarantee and it breaks. Had to clean the tank myself, take the fish out, put 'em in bags and... Where'd the fish go? Come on, Peach! (..)2003 Finding Nemo