I think it probably depends on your goals. As Susan mentioned, there recently have been reports that several shorter bursts of activity during the day can be as beneficial as one longer exercise session, but if I'm not mistaken, this is in terms of general health benefits (lower blood pressure and resting heart rate, reduced risk of osteoporosis, etc.). However, it would seem to me that for cardio endurance and optimum calorie/fat burning, a longer session would be more effective.
Still, getting some cardio in, however long you have, has got to be better than doing nothing at all. Go for it!