since when vs though

since when

adv
  • (as an interrogative interjection) Used to indicate doubt as to the veracity of a statement. 

  • From what time. 

though

adv
  • Used to intensify statements or questions; indeed. 

  • Despite that; however. 

conj
  • Despite the fact that; although. 

How often have the words since when and though occurred in a corpus of books? (source: Google Ngram Viewer )