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]
(..) Hold on, I still need some of your permission slips. And $ for the field trip to the Griffith observatory next weekend. Pay now or forever regret missing out in this experience of a lifetime, guys. You're good. You got it./ Thank you Mr. Wenneck. Thank you, Wilson. Thanks Bob. Way to go. (..)2009 The Hangover