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]
(..) - This is so exciting. You were married here, Your Grace? - Yes. - Must seem like only yesterday. Seems like a lifetime ago. Your husband, was he buried here as well? No, he wanted his remains returned to Storm's End. - Such a tragedy. (..)2013 TvShow: Game of Thrones Title: And Now His Watch Is Ended Season: 3 Episode 4