As an EECS senior at MIT, I haven't measured how many hours a week I work, but it certainly feels like "all the time" is a pretty close guess. I think if you make the startup your priority, and take only the CS requirements (which can easily be done without taking 60-hour-week courses), and you're great at time management, it can work. I've seen at least a couple of cases of this just on my dorm floor in the past couple of years.