On January 1, users discovered that the scheduling portion of the “Do Not Disturb” feature in iOS 6 failed to work. The scheduling would turn itself on as scheduled, but would fail to turn itself off again. Apple now wrote that the “Do Not Disturb” function will resume normal functionality after January 7, 2013.

Symptoms
After January 1st, 2013, Do Not Disturb mode stays on past its scheduled end time.

Resolution
Do Not Disturb scheduling feature will resume normal functionality after January 7, 2013. Before this date, you should manually turn the Do Not Disturb feature on or off.

To turn off the scheduling feature, tap Settings > Notifications > Do Not Disturb and switch Scheduled to Off.

Source [MacRumors]