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]
(..) Hell, you're still so damn young. And what's here for you now anyway? I tell ya. If I could, I'd trade places. A whole lifetime ahead of you with a chance to hit the reset button. In a few years, this might all feel like nothing more (..)2013 TvShow: Breaking Bad Title: Confessions Season: 5 Episode 11