long time vs eon

long time and eon both are nouns.

Word	Noun	Adjective	Verb	Adverb
long time	Yes	No	No	No
eon	Yes	No	No	No

As nouns, eon is a hyponym of long time; that is, eon is a word with a more specific, narrower meaning than long time:

Other hyponyms of long time include month of Sundays, aeon, blue moon, year dot.

long time (noun)	eon (noun)
a prolonged period of time	(Gnosticism) a divine power or nature emanating from the Supreme Being and playing various roles in the operation of the universe
	an immeasurably long period of time
	the longest division of geological time