I know of someone used human public hair removal.... Nair... on her stallion's muzzle, to get rid of his regrown whiskers, for the show season.... not only did she forget she put it on.. she did it the night before a show.. for the first and LAST time (I hope...), his lips swelled up, making him look like he had monroe lips.. Were not sore, just puffed up like a puffer fish... so here is her multiple champion show stallion.... with puffed up lips...
I would not use human hair removal cream on my horse..at all... regardless of how long it was applied and how much you used, as our skin PH levels are very different to horses, and I would not want to risk harming the health of the horse.

hope this helped.