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]
(..) Jump! Come on! Head down! Officer Hopps to dispatch! Are you familiar with Gazelle? Greatest singer of our lifetime. Angel with horns. Okay, hold on. Keep watching. Who's that beside her? Who is it? Wow, you are one hot dancer, Benjamin Clawhauser. (..)2016 Zootopia