nine-day wonder vs seven-day wonder

nine-day wonder

noun
  • Something that generates interest for a limited time and is then abandoned. 

seven-day wonder

noun
  • Synonym of nine-day wonder 

  • An individual or a process which requires only seven days to be perfected. 

How often have the words nine-day wonder and seven-day wonder occurred in a corpus of books? (source: Google Ngram Viewer )