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Hi,
I found that the best compromise between gain and noise, with my camera (unmodded Canon 450D) is at ISO400.
For the length, if you're under light polluted sky, then I guess the best you can do is expose as much as you can while keeping the peak of the histogram at around 75% of the window. Just make sure the right tail doesn't get cut by the right border (in that case you'd be losing information).
So expose as long as you can, as many times as you can. During summer I wouldn't do ISO800 as the noise would be really high.
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