How long has the man suffered from the symptoms he described?

M: Good morning, Doctor.
F: Good morning, Mr. Smith. What can I do for you?
M: I feel there’s something wrong with me. but I’m not sure.
F: Could you explain more?
M: I usually get a runny nose, watery eyes, scratchy throat and other symptoms on Monday. It will last for the following five days. And since I’m terribly busy with my work, I won’t have time to see the doctor. But when I do have time to see the doctor on weekends, the symptoms will disappear all of a sudden.
F: And you will get same symptoms again next Monday?
M: Exactly.
F: How long have you been like this?
M: About one month.
F: Did anything special happen during the past four weeks?
M: Nothing special. Oh, wait! My company has moved from the old address to the present Mount Plaza.
F: Do your colleagues have the same trouble with you?
M: Well, some of them.
F: I see. You must be suffering from so-called sick building syndrome. Some of them can even lead to cancer.
M: That’s terrible! Is there anything I can do to the "sick" building?
F: Don’t worry. I will write you a prescription to ease your symptoms.