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When did the various Traitor Legions start wearing the symbols of Chaos or their chosen Chaos Gods?

I'm assuming the Word Bearers were probably the first to do so.

Definitely by the time of the Siege of Terra for the god specific Legions.

Generally speaking there aren't hard and fast rules.

Those Legions that were more dedicated to the Gods weren't even consistent in the application of symbology. The new Eidolon novel has members of the IIIrd who are fully divulged in the excesses of the Dark Prince with chains, piercings, flesh cloaks and all manner of grotesques alongside those with pristine armour and weaponry.

We have got a recent example of a World Eater with the beginnings of the markings of Khorne:

The Word Bearers covered themselves in devotional script and the trappings of the Octed while officers would wear helms shaped with the visages of Daemons.

By the Siege of Terra, many within the Sons of Horus had begun to worship Horus as a God in his own right and applied the symbols of Chaos to their armour in worship of him:

The Death Guard sort of cheat the system by becoming almost entirely composed of Plague Marines, where the symbols of their God were less painted and more the very flesh on their bones.

Many Knights and Titans among the Traitor forces had become corrupted by the powers they now served, growing horns, spines, claws and tails:

The Thousand Sons didn't really change (haha get it) their armour until the casting of the Rubric of Ahriman.

The Night Lords, Iron Warriors, and Alpha Legion were largely not bothered too much by God-bothering.

Thanks Gert, that's a great answer