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]
(..) Lady Mormont speaks harshly and truly. My son died for Robb Stark, the Young Wolf. I didn't think we'd find another king in my lifetime. I didn't commit my men to your cause 'cause I didn't want more Manderlys dying for nothing. But I was wrong. Jon Snow avenged the Red Wedding. (..)2016 TvShow: Game of Thrones Title: The Winds of Winter Season: 6 Episode 10