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]
(..) It's clear from your vacant expressions The lights are not all on upstairs But we're talking kings and successions Even you can't be caught unawares So prepare for the chance of a lifetime Be prepared for sensational news A shining new era Is tiptoeing nearer And where do we feature? (..)1994 The Lion King