давным-давно – English translation

away back; lang syne; a long time ago; a long while ago; ages ago; in the year dot; once upon a time; a good while ago (Anglophile); way back when (продолжение не требуется, выражение самодостаточно.

thesaurus.com SirReal); from the year one; years and years ago; over a long time ago (Баян); long ago; a really long time ago (VLZ_58); way back in the past (Damirules); in the days of old (Andrey Truhachev); in the days of old (Andrey Truhachev); so long ago (Побеdа); way back in the old days (4uzhoj); gazillion years ago (Raz_Sv); time out of mind; anciently

Игорь Миг

in the old days; all along ago

amer.

back yonder

brit.

yonks (Anglophile); yonks ago (Anglophile)

Gruzovik, inf.

very long ago; ages and ages ago; in a very long time

idiom., south.afr.

before the rinderpest (In South Africa, a long time ago; referring to the 1896 epidemic of the cattle disease, treated as a landmark. // Rinderpest is a contagious viral disease of cattle that periodically caused heavy losses in much of Africa. The 1896 epidemic was so devastating that it was treated as a historical landmark, so giving rise to this expression.

oxfordreference.com 'More)

inf.

ages back (Technical); long long ago; way, way, way back (Lethbridge says his uncle and cousins were travelling near Cartwright in January a few years before the Black Tickle incident when they saw big swirling lights above them for a few moments before they disappeared. "People have reported unusual sightings way, way, way back, and the same as elsewhere in the world," Lethbridge said.

cbc.ca ART Vancouver)

lit.

when Queen Anne was alive

Makarov., engl., inf.

the year dot

obs., idiom.

many moons ago (досл. "много лун назад", т. е. много ночей (и дней, суток) тому назад

cambridge.org Anglophile)

poetic

in the times of yore

progr.

in those far-off times (ssn)