Quiet Hours generally go from late at night till early in the morning, as agreed upon by a dorm so as to provide a safe (or at least relatively noise-free) time for people to sleep in. Regardless of whether it's quiet hours or not, though, it's generally considered poor form to not turn your music down if asked to do so.
In EastDorm, midnight through 10am, or 2am through noon on weekends.
During FinalsWeek, the entire campus is under all-day QuietHours except for fifteen NoisyMinutes per day.