Some experts say you should do cardio 6 days a week (or everyday if that keeps you in a good routine). Six days is for optimum fat burning.
30 min a day is good to start with. But as you progress you'll want to increase that time. You could try starting with 30 min and add 2 min each week. Do a double (60 min) 1 day a week.
Don't forget to add some kind of strength training. You don't even need weights. Start with pushup's, sit ups, lunges and squats. there are others too that don't require anything but your body.
What your portion sizes and stock your dorm room with healthy treats, fruits and nuts. If you have a frig even better.
Good luck. Join us by posting your daily/weekly exercise if you'd like. It's a great way to stay motivated!