Well, I've been told that it's ok to do your work out in shorter sessions. For instance, it's ok to break a 30 minute cardio up into 3-10 minute sessions.
But the almighty Jillian Michaels was asked and here is her answer:
Well, not really, and here's why: The longer the cardio session, the better the afterburn. "Afterburn" is technically known as post-exercise oxygen consumption, and what that fancy term refers to is your elevated metabolism after a workout. Afterburn can last anywhere from 15 minutes to 48 hours, depending on the intensity and duration of your workout — therefore, if you break up your cardio, you'll burn the same number of calories during training, but you'll minimize the number of calories lost because of afterburn.
If you really have to break up your cardio, all is not lost —