It depends on your physical condition. You need to run at a speed where your heart rate is in the target range for your objective for at least 40 minutes. See your doctor to determine the target...
You lose weight but burning more calories than you consume. You would want to exercise within your ability and for the amount of time to burn the calories you want to
You should run intensely for 30 minutes, because after thirty minutes the body goes into a state were to keep fueling you it will store body fat, resulting in you not losing nearly what you thought...