A.Z. Central explains that how long it will take a person to achieve flat abs depends on the fat content in his body. First, a person needs to use a body mass index (BMI) calculator to determine the amount of fat he has in his body.
One pound of fat is 3,500 calories, so lowering caloric intake by 500 calories a day loses 1 pound each week towards flattening the stomach.
According to Registered Nurse Diane Marks for A.Z. Central, the Centers for Disease Control and Prevention reports that if a person's BMI is over 24.9, that person is overweight.
Once a goal weight is established, the length of time that it will take to achieve that goal can be determined. If an individual has a goal of 140 and he weighs 160, it should take him approximately 20 weeks to lose the fat.