I don't think so vs when Hell freezes over

I don't think so

phrase
  • I think what has been said is untrue. 

  • No, absolutely not (used to refuse or decline something proposed). 

when Hell freezes over

adv
  • Never; not in this lifetime or a time in the unforeseeably distant future; not a chance. 

How often have the words I don't think so and when Hell freezes over occurred in a corpus of books? (source: Google Ngram Viewer )