long time vs period of time

long time and period of time both are nouns.

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

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

Other hypernyms of long time include period, time period.

long time (noun)	period of time (noun)
a prolonged period of time	an amount of time