As long as you haven't 'set' the pectin in the juice by boillng it, you shouldn't need to add pectic enzyme at all...it WILL clear on its own, as Denny points out.
The juice from which you're making the cider should never be boiled. I've never used pectic enzyme, but I believe that it is generally added at the beginning of fermentation...bujt again, never having used it myself, I can't be sure about that.