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]
(..) - Keep moving, frat boy. - Come on. Out with it. - Don't rush me. l mean... You shouldn't be impatient with the opportunity of a lifetime. l'm gonna sit down because you're so sexy? My knees are weak. Really? Dr Cox? l know l'm being annoying, (..)2001 TvShow: Scrubs Title: My Mentor Season: 1 Episode 2