I'd imagine we might see random npcs shout "YOU!!!" or something. It probably won't make sense no matter what they do. I just hope the story doesn't try to take itself too seriously. That would make it all better tbh.
I'd say just before Legion. It wouldn't make sense for the whispers to come to Vol'jin so far after their plan had begun. Though why Sylvanas didn't orchestrate to become warchief above Vol'jin is yet to be seen. Yes she needed the opportunity, but that kind of thing can be artificially induced.