Like since the crash. There seem to be some lingering camera pans and shots of Locke that don't really make sense for some certain episodes as a standalone. And sometimes Locke's personality would. . . shift and seem out of character; not completely taking over, but sort of coming through a bit, ya know? Have the creators been planning it from the beginning? I dunno, maybe I'm just crazy.
And I know the Man in Black didn't use Locke's body after 316 crashed, but maybe he was using it earlier to gain knowledge of his future facade.