Board Thread:Warhammer 40k Novels/@comment-28693258-20180914124152/@comment-2125941-20190126002331

If the emperor predated the birth of the dark gods in "reality" it would explain how he could learn so much about the warp without being effected by the influence of the gods.

So i would say he is ancient.

Chaos grew to significant power by the time of the rise of Slaanesh in early 30th (it's complicated because time doesn't exist in the warp). However Khorne the second oldest god is born somewhere in middle ages as far as i know and Nurgle predates him so i would say the Emperor must be born before Christs birth at least.