lifespan vs period of time

lifespan and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
lifespan	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of lifespan; that is, period of time is a word with a broader meaning than lifespan:

lifespan: the period during which something is functional (as between birth and death)
period of time: an amount of time

Other hypernyms of lifespan include period, time period.

lifespan (noun)	period of time (noun)
the period during which something is functional (as between birth and death)	an amount of time