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 the... Those girls sold me cookies! Do you have any idea how lucrative this moon heist could be? I give you the opportunity of a lifetime, and you just blow it! - No, I didn't. - Oh, really? You just wait until Gru sees my latest weapon. (..)2010 Despicable Me