life expectancy vs lifespan

life expectancy

noun
  • The amount of time something is expected to last. 

  • The amount of time one is expected to live. 

lifespan

noun
  • The length of time for which something exists or is current or valid. 

  • The length of time for which an organism lives. 

  • Some clams have lifespans far longer than those of human beings. 

How often have the words life expectancy and lifespan occurred in a corpus of books? (source: Google Ngram Viewer )