If you are only eating 500 calories a day, it won't take long to lose a pound. A single pound is equal to 3,500 calories. If you know how many calories you normally eat, subtract the 500 from that. Take your answer and divide that into the 3,500 and you will see how many days that will take. I don't recommend only eating 500 calories though. I don't think that is enough to stay healthy. Try adding more exercise to your day to burn off calories.