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]
(..) - Good. - Would you like some water? - Yes, please. You know, Nicole, this crime is forcing me to use all my knowledge. Everything I have learned in a lifetime of investigating is coming into play. Do you mind if I have these? I haven't eaten. Oh, please, go ahead. Things are appearing in my mind (..)2006 The Pink Panther
(..) It's true. You see... ...I have waited a long time to prove myself. And I've been given the opportunity of a lifetime. And I intend to show the world exactly who I am... ...and what I can do. I must not fail. You won't. I know it. (..)2006 The Pink Panther