He does other things too. But in the time between these other things he smokes cigarettes.
There is a certain denial of common logic he must engage in order to habitual smoke cigarettes. Cigarettes, in excess, are lethal. They cause the deterioration of the lungs and the throat. They are carcinogenic and cancerous. Eventually they will kill the body. Smoking habitually is, in essence, to deny this fact. He is aware of this denial, yet he smokes.
His thinking follows thusly: cigarettes are deadly, to this he assents, but the lethality is relative to time. From this premise there are two logical positions. He can tell himself that he will, eventually, quit; at a certain point he will give up cigarettes (presumably before they become lethal); or he may deny his fear of death. In this method he will not quit smoking. He will assume that cigarettes will cause his death. This requires the additional premise that life in general is lethal relative to time, and so a death from cigarettes is neither unnerving nor unreasonable. Some days he prefers the former reasoning, some days the latter.
Regardless of whether or not he will quit in the future, he does still smoke now. He is afforded a reason to leave his room during long stretches of work. When he has a cigarette he does not feel as awkward waiting for the bus or standing around, wasting time, before class.
In this way his activities determine how many cigarettes he smokes. Some days he smokes as few as two or three, other days he smokes a pack. His cigarette intake is directly proportional to the amount of time he spends outside his room. For instance, on days in which he has classes, he smokes a cigarette while waiting for the bus, another before his first class, then another after his first class. Then, on his way to the second class, he has another, and before the third class, and the fourth. Then while waiting for the bus, and another after dinner.
When he drinks heavily, he smokes heavily, but the next day, when he is hung over, he does not smoke as many, as he feels both sick and the desire not to leave his room.