Good question MrGoldfish. It really depends.
This one was shot with an 8mm Fisheye Lens at f5.6, iso400, and a 30 second shutter time. There was a bright moon and I used a small flashlight to paint the foreground a little.
You can usually use a 25 to 30 second shutter speed with a wide angle lens, but as your focal length gets longer (zoomed in) the movement of the stars becomes more noticeable. At 25 seconds the stars are already elongated.