I've never heard of willpower depletion. I've heard people say that they don't have enough willpower, but not that they're out of willpower. Surely willpower is a long-term stat like CON, not an diminishable resource like HP.

I've never thought that I've had much willpower (possibly a nocebo effect originally generalised from a few early cases?). But on those occasions where I have used my willpower, this has always made subsequent uses easier. I can't imagine using it up.