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]
(..) - Well, at home in the garage, I had-- How many people did you cut up with it? Or with the couple hundred knives you've owned in your lifetime? Your logic is backward. You can't ignore the disappearance and the trunk and the jewelry. I checked the railroad station. He bought a ticket. (..)1954 Rear Window