two weeks vs period of time

two weeks and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
two weeks	Yes	No	No	No
period of time	Yes	No	No	No

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

Other hypernyms of two weeks include period, time period.

two weeks (noun)	period of time (noun)
a period of fourteen consecutive days	an amount of time