What shutter speed should you use at night?

The basic answer is as slow of a shutter as possible without blurring the picture. With no flash and no tripod I am not steady enough to get a sharp exposure at much less than 1/80th of a second. If there is a tripod involved things change. For pictures of people you might get away with as low as 1/40th of a second, but the people would have to know to hold more still than usual for the picture. 1/30th or 1/20th might be used with a flash, where you wish for dim lights in the background such as city lights are desired to be seen, the flash will expose the people and the shutter being open will give the background more time to come through. On a tripod I've exposed as long as 30 seconds, for star field pictures, or interesting pictures of buildings only lit by small security lights or moonlight. You can take a great picture in what seems like darkness with a tripod and remote, even better with a camera with mirror lock-up to get really sharp long exposures.