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]
(..) Isn't it a greater crime to starve a family? Go on, Locksley. Go on. I've seen enough blood spilt to last two lifetimes. Get off my land. Tell Nottingham what happens when his scum pick on children. You travel miles to save my life then leave me to be butchered?! (..)1991 Robin Hood: Prince of Thieves