Variety is the spice of life, and it is my belief that a person can wear skirts at any length they choose - as long as it is long enough to cover what shouldn't be on display and short enough that one doesn't trip over the hem.
I've read of different formulas for figure flattery such as the 'golden number': length of body shoulders to toes, including heels / 1.618 = golden number (hem at this number down from shoulders) - which seems to be just above the knee.
Another suggestion is to hem below the knee at the top of the curve of your calf muscle.
At school 'back in the day' a skirt had to be no shorter than finger tip length when your hands were resting comfortably at your sides (or else you would be sent home.)
I learned the way to hem a kilt the proper length is to have the wearer (male) kneel down on both knees and hem the material such that it just brushes the ground.
For a long time I liked swooshy midi's that fell below the calf about 6 inches above the ankle bone, but these days I like my dress and skirts best if the hem is mid knee or just below the knee cap. It just looks best to my eye.
What about you? Do you adhere to any 'rules'? What is your preferred skirt length?