Late 1912 would be among the earliest. Of course, German and French scientists didn't think much of Piltdown; English scientist like it (sort of an "Out Of England" movement.) The British Museum didn't allow the bones to be examined for some years for some reason (I don't know why) so it was harder to find out what was actually going on.
No Creationists worked on proving a hoax. That was done by scientists using scientific methods.
What is the Creationists evidence that Piltdown was a fake? Scientific evidence is based on radioactive dating and the fact that Piltdown doesn't fit evolutionary theory.
The point isn't who did or did not expose the hoax. The point is how well it was received and published in the scientific community before it was exposed. IOW, that community is not incapable of deceiving itself, and others for that matter. I can think of no other fields of philosophy where hoaxes have reared their heads except in the one that attempts to sew the head of Darwin on its shoulders.
But how long did it take to expose the Piltdown fraud? Why are Lamarckian drawings still being pushed in some quarters as truthful, a.k.a. "scientific," representations of evolutionary development?