Am on atkins, how long should it take me to lose 15lbs?

Am 5 ft 4 and 135 lbs, want to lose 15-20 lbs, work till 7:30 so dont have much time for exercise, walk for around 30 minutes a day, and run a mile around once or twice a week. I dont want to rush the weight loss, I just want a rough guideline of how long it should take me. Anyone know?