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]
(..) And? And? Happy Hanukkah. - Very funny. - Go get yourself something nice. Lufthansa should have been our ultimate score. The heist of a lifetime. Six million in cash. More than enough to go around. Yo, Stacks! Open up! What is with you? He isn't ready. (..)1990 Goodfellas
(..) Do you understand? There is millions in there. I've been cultivating this son of a bitch for two years. He owes me grand. Once in a lifetime. I could retire. No more nut every week. No more bullshit. My dream comes true. Let's get a drink. These are the guys Jimmy put (..)1990 Goodfellas