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]
(..) That was certainly most unexpected. Thank you. I've always believed in numbers. In the equations and logics that lead to reason. But after a lifetime of such pursuits... ...I ask, what truly is logic? Who decides reason? My quest has taken me through the physical... ...the metaphysical... (..)2001 A Beautiful Mind
(..) Do you see what they're doing there? Congratulations, Professor Max. Thank you, sir. Thank you. It's the pens. Reserved for a member of the department that makes the achievement of a lifetime. Now what do you see, John? Recognition. Well done, Professor. Well, try seeing accomplishment. (..)2001 A Beautiful Mind