at length vs lastly

at length

prep
  • For a long time. 

  • At last, finally, eventually. 

lastly

adv
  • Used to signal that the speaker is about to yield control of the conversation. 

  • Used to mark the beginning of the last in a list of items or propositions. 

How often have the words at length and lastly occurred in a corpus of books? (source: Google Ngram Viewer )