Depends on your body's response to cardio I would think. If the intensity is still high and your body has a good cardio response, I would think it would be enough.
What are you accustomed to doing? If you've been doing 5 days, 60 minute, high intensity cardio workouts and were losing weight, I would think you could maintain at 30 mins. without changing frequency and intensity. Your weight loss would be much slower or non-existent, but you'd just have to experiment with that. You wouldn't want to change more than one element of your workouts though.