How much time does it take to lose a pound?

Depends on diet, exercise, starting weight and metabolism. Most diet strive for 1lb per week. To do this, basically you must reduce your daily calorie intake to 500 calories less than your maintenance calorie burn. If your body needs 2500 calories per day to maintain weight, if you reduce your calorie intake to 2000 calories, you will be 3500 calories short for the week, which is approximately 1 lb of fat.

I once went on a 1000 cal per day diet, which is dangerous, I lost 3lbs per week.