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]
(..) Presented them to Lady Catelyn as a gesture of goodwill from Tyrion Lannister. Seems like a lifetime ago. Do give Lord Tyrion my best when you see him. I was sorry when he died. Your father and I had our differences, (..)2017 TvShow: Game of Thrones Title: Stormborn Season: 7 Episode 2