~0.00006% is the rate of occurence of the phrase ‘once upon a time’ in Google books Ngram Viewer when it peaked around 1922. The link also includes the phrase ‘dark and stormy night’ in reference to the much joked about opener ‘It was a dark and stormy night..’. In­ter­est­ing­ly the maligned dark and stormy opener has never occurred as often as the apparantly much more common phrase ‘once upon a time.’