The French sentence does not speak to how much is in the box. The English sentence does not speak to how much is in the box. Presuming one eats some of the contents of the box on a regular basis, it will last for one week. It is presumed that you start with a full box and eat some of it until the box is empty. This process takes one week. The point is that in English, one does not say "a box of cereals", but "a box of cereal".