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]
(..) I'm feelin' you're being neglected. I'm expecting somebody. Instantly? No, but any minute now. Any minute? Some people live a lifetime in a minute. What are you doin' right now? I'm waiting for him. Would you mind if we waited with you, just to keep the womanizers from bothering you? (..)1992 Scent of a Woman