Tecnically you can use this stuff right away. There is nothing wrong with the hair on the pieces, what you're doing is allowing the hide to dry nicely and absorb any fat or oil that may seep from the drying hide. Borax or cornmeal is merely a means to remove most of the moisture left in the hide, so your retriever doesn't immediately yank it off your desk and head for the backyard where he can bury it.

A dry hide is also one whose fragrance has been minimized, so you might be allowed to keep it indoors when your spouse finds it drying on your tying table.