Here are some suggested exposure times for different types of subjects:
Christmas lights: 1/4 to 3 seconds (pictured) of exposure.
Cityscapes: 2 to 30 seconds of exposure.
Moving water or fountains: 2 to 5 seconds of exposure.
Portraits: 3 seconds with flash. With some HP cameras you can use the Night Portrait shooting mode, which automatically adjusts your shutter speed for night shots and adds a quick flash at the end of the exposure to capture your subject in detail.
http://www.hp.com/united-states/consumer/digital_photography/take_better_photos/tips/night.html