Personally, I wear them a little shorter than the conventional advice for pants. For example, with flat sandals, I prefer to wear long skirts around the anklebone, so that the sandals show. (But I have seen others wearing skirts that brush the foot or even hit the ground...) A Google image search for "maxi skirt and heels" shows people wearing them at anklebone length even with heels, but that looks funny to me -- if I wanted to wear heels, I'd go with a midi skirt for balance, or maybe wear the skirt right down to the floor, like Sandy suggests. The best length might also depend on whether the skirt is flowy or slim-cut.