running time vs period of time

running time and period of time both are nouns.

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

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

Other hypernyms of running time include period, time period.

running time (noun)	period of time (noun)
the length of time that a movie or tv show runs	an amount of time