how long vs long for

how long

adj
  • Used literally: what duration. 

  • Used literally: what length. 

adv
  • Used literally: for what duration. 

long for

adj
  • likely to remain for a long time 

verb
  • To have a desire for (a thing or an event). 

  • To miss (someone). 

How often have the words how long and long for occurred in a corpus of books? (source: Google Ngram Viewer )