It takes 30-40 minutes to roast potatoes in a 375-400 degree Fahrenheit oven. Some recipes recommend shaking the baking sheet at regular intervals to achieve maximum and even crispiness.
The type of potato used doesn't have an affect on cooking time. Before roasting, potatoes should be cut into evenly sized cubes in order to insure that they cook through evenly. The smaller the cubes, the less the cooking time needed. They should also be coated in a browning agent, such as olive oil, to help ensure the ideal coloring. When placed on a baking sheet, potatoes shouldn't be placed too close together.