May your reign last for a thousand, nay, eight thousand generations, until pebbles grow into boulders and moss covers them.
eight thousand years
Original literal meaning; now mainly encountered in classical contexts or as an etymological note.
八千代という言葉は、文字通りには八千年を意味する。
The word 八千代 literally means eight thousand years.
永遠 is the standard modern word for eternity, while 八千代 is a poetic, classical term used mainly in wishes for long life or traditional verse.
千代 also means 'a thousand years/eternity' and appears far more often in poetry and personal names than the rarer 八千代.
for millennia and millennia; forever and ever
From 八 (eight) + 千 (thousand) + 代 (generation/age), literally 'eight thousand generations', used hyperbolically to mean an immeasurably long time.