The Shulchan Aruch does not discuss the length of women’s skirts. Nor does the Shulchan Aruch bring down the halacha of “shok b’isha erva.” Rather the Bais Yosef (Orach Chaim 75) holds that shok is just an example of a body part that is normally covered. The Shulchan Aruch (and the Rambam and others) hold that any body part that is normally covered but is now uncovered is a problem of erva and other people cannot daven in front of it. So according to the Shulchan Aruch (others disagree) whether or not legs are erva depends on the norms in the community in which you are located. See also, Aruch Hashulchan 75.
Now, there is a separate halacha of tznius norms called Daas Yehudis. This is the tznius practices of b’nos Yisroel. The Mishna in Kesubos lists a couple of such practices but does not include covering legs. Therefore, whether and how much legs NEED (obviously a person can choose a higher personal standard of tznius) to be covered (and the corresponding skirt length) for tznius reasons depends on the norms of Jewish community in which you live.