for keeps vs until hell freezes over

for keeps

prep
  • Permanently, so that one can keep something indefinitely. 

  • With an agreement or intention to retain what one gains or receives. 

  • With a strong competitive resolve to win or succeed, as in sports or business. 

until hell freezes over

prep
  • forever; without success, no matter how long one perseveres 

How often have the words for keeps and until hell freezes over occurred in a corpus of books? (source: Google Ngram Viewer )