For a full frame DSLR, the typical rule of thumb is 1 over your focal length. For example, at 50mm, the recommended minimum would be 1/50. Many DSLR lenses (and some cameras) have image stabilization / vibration reduction / optical stabilization. Each mfg calls it something different. With good stabilization, you can shoot that 50mm lens down to maybe a half second.
The gimbal on the Phantom is a similar form of stabilization. I've seen 2-3 second night photos that appear very sharp. Of course, it's very dependent on wind conditions and how many satellites you are connected to, and maybe even how good your IMU and compass calibrations are.
My opinion is anything longer than 3 seconds under ideal conditions likely won't be super sharp...