I shoot star trails with the lens wide open (f/4 or 2.8). The amount of exposure will dictate the length of the star trails. Even a short 5 or 10 minute exposure will give short but effective star trails. Two hours or more will give rather long star trails, but lower the contrast (i.e., the trails will appear dim in the print). It comes down to what you want. If you have the time, shoot multiple shots doubling the exposure each time starting with 5 minutes. A new moon and a dark area sound good. If you want only star trails, then compose the shot without anything else in the frame. On the other hand, you might want to include a piece of the landscape in the frame. Or, you can shoot a few frames and do both. If there is a chance of vehicle lights falling on something in your shot, have a ball cap or something handy to cover the lens until the vehicle passes by. Be careful not to hit the camera.