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]
(..) Rich just means overpriced I can't sleep on a plane so I did my homework -OK, let's focus! What's our goal? Figure out if this is a deal of a lifetime or... everyone knows here something we don't and we're gonna get royally screwed Right, remember that when you meet with guys from Bear! (..)2015 The Big Short
(..) Look Ben, the system fucked up in a big, big way here, OK? And somehow... we , eh.. know before anyone else. You know, this is a once in a lifetime deal! I'm thinking OK, I'll call Deutsche Bank Thank you Ben! Oh and Bear, what about Bear? They have some apprehensible product! (..)2015 The Big Short