How long it will take you to slim down depends on two factors: how hard you want to work and how much weight you have to lose. The Centers for Disease Control and Prevention fitness guidelines for adults recommends two days of strength training per week, combined with either vigorous aerobic exercise of least 75 minutes a week or moderate-intensity exercise of at least 150 minutes per week to maintain weight and experience the health benefits of exercise. Increase the time you spend exercising beyond this basic level, and modestly restrict your calories to slim down in a predictable, healthy way.
Doing the Math
Figure out how much weight you want to lose by using height and weight charts to set a goal weight. For example, a 5-foot 5-inch female with a small frame weighing 160 pounds would have to lose 30 pounds to reach a goal weight of 130. Because 1 pound equals 3,500 calories, you'll need to burn 105,000 calories to lose 30 pounds. Once you know how many calories you'll have to cut, you can create a program of diet and exercise to achieve your goals.
Another factor you have to consider when slimming down is how hard and how often you want to work out. Find one or more aerobic activities you like and work out consistently every week. Use a calorie burn calculator, such as that available on the website Health Status, to estimate how many calories you'll burn while exercising to figure out how long your weight loss will take you. If, for example, you increased your workout time to 150 minutes of aerobics at a vigorous rate, a 160-pound woman would burn an extra 636 calories a week. To lose 30 pounds, it will, therefore, take her three years without any other intervention.
You can speed up your weight loss significantly by cutting calories. Small changes recommended by the United States Department of Agriculture include cutting out trans-fats, eating more fresh fruits and vegetables, choosing lean meats and fish and switching to low-fat dairy products. For example, a 160-pound moderately active woman is probably eating about 2,000 calories a day to maintain her current body weight. By making just subtle changes she can cut down her calories by 500 a week, which would translate into a 1 pound loss every 7 weeks. By combining diet and exercise, her total caloric deficit per week is now 1,136, meaning she'll lose 30 pounds in just under two years.
Don't try to restrict your caloric intake to less than 1,500 calories per day because diets that are too restrictive actually slow down your metabolism and halt your progress, according to the website Fitness. See a doctor first if you suffer from medical conditions, such as diabetes, arthritis, lung or kidney disease or obesity. Inactive people or women over the age of 55 should get medical clearance before starting an exercise program, according to MayoClinic.com.