eternity vs uncertain period

eternity

noun
  • A comparatively long time. 

  • Existence without end, infinite time. 

  • A period of time which extends infinitely far into the future. 

  • The remainder of time that elapses after death. 

  • Existence outside of time. 

uncertain period

noun
  • An unspecified amount of time 

How often have the words eternity and uncertain period occurred in a corpus of books? (source: Google Ngram Viewer )