As a "rule of thumb" I have found that for a standard 50mm lens, with the camera on a tripod, at ISO 100 or 200, that f/8 or f/16 at 30sec is about right for a lot of situations where there is ambient street light, in the city. If you are going out in the area's where there is no light then that is a different matter.
Of course this depends on the camera you are using, what it's sensor is like and so on. For film, be it b&w, trannie, neg... I bracket my shots. For example. I set the camera shutter to 30 sec exposure, then shoot 3 shots, one at f/16, then the next one at f/8 and another one at f/5.6. This is for city shooting. Then I move onto the next photo...
Now if you are looking at doing star trails and so on, this is a different story again. I had some ammazing shots using my non Nikon camera (medium format) that I left open for 2 hours at f/16 in the middle of the bush during the part of the month with no moon in the sky. The light trails are spectacular and well worth the effort.
Doing such long exposures with a digital camera and using noise reduction is a lot more complex. Suffice to say that taking a shot for 2 1/2 hours then waiting another 2 1/2 hours for the noise reduction would not be an ideal situation.
As far as I know there is a program out there for people who like to do star trails with digital. Correct me if I am wrong, but you take a series of photos, 10 or 20 or 30 sec long over a period of time, then you put them in the program, and it combines them all into one image. I have never used this, but maybe if someone here has, they could tell us about it.
Have a great day everyone. Oh, night shots are fantastic.