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]
(..) - You're too modest, Mr Holmes. - I'm really not. Got yourself a fan. Tell me more. That's all you're going to know. In this lifetime. Why did he do that? Why did he have to leave? You know him better than I do. I've known him for five years and, no, I don't. (..)2010 TvShow: Sherlock Title: A Study in Pink Season: 1 Episode 1
(..) Seen a lot of injuries, then. Violent deaths. Well, yes. Bit of trouble too, I bet? Of course. Yes. Enough for a lifetime, far too much. - Want to see some more? - Oh, God, yes. Sorry, Mrs Hudson, I'll skip the tea. Off out. - Both of you? (..)2010 TvShow: Sherlock Title: A Study in Pink Season: 1 Episode 1