Even twice weekly exercise benefits health

According to a ReutersHealth article:
Now, researchers report that healthy men who burned off at least 1,000 calories during one or two weekly bursts of activity were 60 percent less likely to die over a 10-year period than sedentary men, who expended less than 500 extra calories per week.

"Physical activity is good for health -- anything is better than none," study author Dr. I-Min Lee of Harvard University in Boston, told Reuters Health. "If healthy, and all you can do is one to two bouts a week of activity, go for it."
The longevity benefit of occasional exercise did not extend to those who had underlying health problems like obesity, smoking, or hypertension.

However those with these underlying health problems who exercise 4 to 5 times a week or more do live longer.

Study authors speculate that the protective effect of exercise in people with an underlying health problem (obesity, smoking, high blood pressure) only lasts a day or two, making more frequent exercise important for these people.