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]
(..) An armed team has taken over the White House. Do we have the president? I spoke to Walker. He had eyes on Eagle. Fifteen minutes is a lifetime. Radio chatter indicated they were headed for the PEOC. No contact. We have to assume they've been compromised. We gotta get you to a phone. (..)2013 White House Down