There is not a specific length that your dog leash should be. It all depends on what you want and the personality of your dog. However, as a general rule, 10 to 15 feet is always a great length, as you will be able to give your dog some freedom but also control it from afar.
Your dog's leash should be long enough to where it is not always tight. Keep in mind that you want your dog to feel free. Try to keep the leash loose and let your dog think that it is in control. You and your dog will both have more fun.