shelf life vs period of time

shelf life and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
shelf life	Yes	No	No	No
period of time	Yes	No	No	No

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

shelf life: the length of time a packaged food or drug will last without deteriorating
period of time: an amount of time

Other hypernyms of shelf life include period, time period.

shelf life (noun)	period of time (noun)
the length of time a packaged food or drug will last without deteriorating	an amount of time