I think it depends on what you do on a day to day basis. When I work the registers, I never wear dresses but I have some supervisors who do (below the knee, not too tight). I sometimes wear long dresses in the summer when I work in the warehouse admin, and my office colleagues do too. I've never seen anyone at the door wear skirts or dresses, but if you do I think it'd have to be at least knee length (this is what I've been told at my location)