A good pair of dress trousers needs to break over the top of your shoe, which means they should crease slightly once before they meet your footwear. A general rule is that they are too short if any part of the sock can be seen when you walk.
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Too long
Too much length is just as bad as too little. If the back of the trouser leg trails on the floor as you walk around, it will look unsightly and wear away the hem. This can be prevented by raising the hem so it clears the floor around your heel when you have no shoes on.
Bottom line
Dress trousers are meant to provide a well-tailored image so a happy medium between too short or too long is the ideal. Most aim for the trouser hems to be slightly longer at the back than the front so they break once over the front of the shoe.