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]
(..) What's the matter? Are you too scared to fight back? I promised I wouldn't. Stop it! Don, stop! While I took the beating of a lifetime, Don Price was ultimately defeated. All the physical activity had worsened a congenital valve defect. Put simply, his heart wasn't strong enough. (..)2003 Big Fish