How long does Mr. Kapasi calculate it will take before he gets a letter from Mrs. Das?

In Jhumpa Lahiri's short story "Interpreter of Maladies," Mr. Kapasi, a tour guide, develops an affection for Mrs. Das, an Indian-American woman visiting India with her husband and their young children. After Mr. Kapasi takes pictures of the family, Mr. Das takes a picture of Mrs. Das and Mr. Kapasi sitting next to each other. At this point in the story, Mrs. Das requests Mr. Kapasi's address so that she can send him copies of the photographs, inspiring in Mr. Kapasi a fantasy of their future as pen pals.
This fantasy is so important to Mr. Kapasi that he makes an effort to calculate when he will actually receive a letter from Mrs. Das. In order to make this calculation, he first asks Mrs. Das when the family will return to America. When she responds that they will return in ten days, he imagines her needing a week to "settle in" and another week to "develop the pictures." Then he predicts she will need "a few days" to actually write the letter, and then the letter will require at least two weeks to travel to India from America. He concludes from these calculations that he will receive the letter and pictures "in approximately six weeks' time."