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]
(..) #Honey, happy birthday. And this is ours. It is no dream. Lower your expectations. No, but you need it. It is a ... #You have to have! You need it. - A lifetime ahead. - And the shirt is blue. Dance ... job interviews ... I'm going to wrinkle. Mason. #Come here, son. (..)2014 Boyhood