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

M: Good morning, Doctor.
W: Good morning, Mr. Smith. What can I do for you?
M: I feel there’s something wrong with me, but I’m not sure.
W: 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!
W: And you will get same symptoms again next Monday?
M: Exactly.
W: How long have you been like this?
M: About one month.
W: 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.
W: Do your colleagues have the same trouble with you?
M: Well, some of them.
W: I see. You must be suffering from so-called sick building syndrome.
M: Sick building syndrome? You mean the building got sick and we have been infected?
W: You may put it that way. When a new building is just finished, the chemical vapors being given off by the glue, paint and other construction materials and moisture can’t escape, the building, just like people, can’t breathe properly. So it gets sick.
M: And these chemical vapors may be very irritating to people.
W: Some of them can even lead to cancer.
M: That’s terrible.