If i brisk walk for 40 minutes everyday (at 15mins/mile) and restrict my calorie intake to 1300 per day, how long would it take to lose 15 kgs?

Slow & steady - 6 mo: A rule of thumb for calories burned walking or running is 100 cal/ mile. At your pace, it is about 300cal/day. You want to loose about 33lbs which is a net caloric deficit of 115, 500 (33lbs x 3500cal/lb). To determine how long, one also needs to know your current wt. Another rule of thumb--maintenance of body weight is about 100 cal/ lb. A safe goal is 1-2 lbs/ wk.