enlistment vs period of time

enlistment and period of time both are nouns.

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

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

Other hypernyms of enlistment include period, time period.

enlistment (noun)	period of time (noun)
the act of enlisting (as in a military service)	an amount of time
a period of time spent in military service