Right after the Age of Strife where Warpstorms are everywhere and cut off a lot of planets from communications and space travel, splintering the DAOT-era humanity into scattered remnants of their former selves.I'd say it started even earlier than that, when he decided to unify the galaxy through military conquest.
And that's when Big E started taking over Terra with the Thunder Warriors and also eliminated them discreetly when their usefulness is over to replace them with Space Marines and Custodes.
Along with Primarchs as well until they got taken and sent across the galaxy to various planets they grew up in by Chaos Gods through Erda, the perpetual woman who take personal issue of Big E and His methods.