If you walk at a pace of 3 MPH for 30 minutes you should burn off 149 calories per 30 minutes of walking.
If you walk an hour at 3 MPH you will burn off 298 calories which means in one month you should drop 1 to 2 pounds of weight in 1 month and in 2 months you could drop 5 lbs or even more depending on how much you walk per day.