At a rate of 1 to 2 pounds per week, it would take approximately 10 to 20 weeks to lose 20 pounds. Although this may seem slow, losing weight at this rate helps keep the weight off permanently, according to the Centers for Disease Control and Prevention.
To lose 1 to 2 pounds a week, figure out how many calories are burned every day. Then subtract 500 to 1,000 to find out how many calories should be eaten per day to lose weight. Meanwhile, stay active to boost the metabolism, as it might slow down in response to the calorie reduction, according to the Academy of Nutrition and Dietetics.