How long will it take to loose 20 pounds?

im 160 pounds 5'4 and im 15. my nutriotionist or watever he is said that if i get down to 140 ill hav a rele low body fat and have abs and ****. if i eat good and weight lift and walk on the the tread mill for 30 minutes and do the stepper for 20 every day how long will it tak to get off?