long since vs long ago

long since and long ago both are adverbs.

Word	Noun	Adjective	Verb	Adverb
long since	No	No	No	Yes
long ago	No	No	No	Yes

As adverbs, long since and long ago are synonyms defined as:

Other synonyms of long since include lang syne, langsyne.

long since (adverb)	long ago (adverb)
of the distant or comparatively distant past	of the distant or comparatively distant past