I think it is really difficult to say how many days you will need. You should try to exercise regularly and to have smaller portions of food. But nevertheless I think it will take you at least 20 weeks. My doctor told me that you should lose about 0,5-1 kilo a week. During the first weeks you will lose more, but then there will be a period of time when you will lose less. But don't get frustrated during this time. That is normal, because you only lose water during the first days!