time off vs period of time

time off and period of time both are nouns.

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

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

Other hypernyms of time off include period, time period.

time off (noun)	period of time (noun)
a time period when you are not required to work	an amount of time