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]
(..) I am prepared. We all think that at first. We are all wrong. How is it you know this place so well? A lifetime ago, I, too, sought the stones. I even held one in my hand. But it cast me out, banished me here. Guiding others to a treasure (..)2018 Avengers: Infinity War