With a proper and controlled low calorie diet along with that regular physical exercise ,one may loose about 2-4 kgs a month and thats a healthy as well as a natural way to loose weight.So accordingly 4-5 months time will be required to loose 20 kgs.In terms of diet,it should be low in fats,simple sugars and even salts.Whereas moderate in terms of proteins.Any kind of physical exercise (especially cardio) like a brisk walk,swimming,jogging,cycling for 40-45 minutes is a must