From a male perspective, for a woman a lot depends on body and leg shape. Great body and long shapely legs it would be hard to argue with any length, however short. For most mid knee to 3-4 inches above, depending on leg length. Ugly knees - mid calf tartan skirt.

I was recently watching a TV program on which most of the women were wearing short to very skirts or dresses and commented to my wife that it must have been the lighting which made their knees look so awful. Then there appeared one whose knees looked really nice. Enough said, maybe the lights didn't help but I would have sacked most of the dressers.