How much time will it take a slim 20 year old female to tone up?

Exercise!: Biking, swimming, going to the gym or enjoying a brisk walk, dancing, sweeping or raking can all be good exercise. Exercise is best done by routine, on a schedule to build it into your daily life rather than getting around to it which tends not to work. I would say that "toning up" could start in as little as a week or two, but a steady regimen is needed. Don't worry about time.