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]
(..) whatever game she's playing, she's playing for us. I'm commander Paylor of district . For the first time in our lifetimes, we're standing together with districts. From what I see here, we've already made history. But history doesn't stop (..)2015 The Hunger Games: Mockingjay - Part 2