First of all, your camera has certain limits set in place by the manufacturer, but you'll generally not want to push those, so that you can get better quality photos. So although you might have an ISO HI 2 setting, it's going to be super noisy. Generally, you'll want to stick to as low an ISO as you can get, without compromising other elements of your photo.
If your subject(s) are truly not moving (e.g. rocks, grass at longer distances, other solid objects), you can literally put your camera on the tripod and let it run for a few minutes (or even hours) if you need to. In this case, a low ISO would be better because you'll have less noise, without compromising your image. Note that I'm not going to get into lighting here, but be wary of "tiny" lights that are brighter than everything else -- they'll definitely show up in your images.
However, some "stationary" objects actually move slightly. A good example of this would be trees. While they don't generally get up and walk, you might be surprised how much the branches sway in the wind. If it's completely calm outside, you might be able to ignore this, especially if the trees are further away. But if there's even the slightest touch of breeze, you'll want a significantly shorter shutter time so as not to get blurred tree outlines. In that case, a higher ISO would be justified. They make noise reduction tools for a reason.
One other field to be wary of is the sky. Since we're rotating, the stars aren't going to stay in the same positions, so even relatively short exposures of ~10 seconds could potentially be problematic. (A general rule is about 1 pixel of blur for every 4 seconds of exposure with a wide angle lens at 24MP). For this area though, I'd just encourage you to try it out once and see how much of a difference it makes in your photo. It may not be noticeable for what you're using it for (or it may!).
In short, the lower you can get your ISO without compromising your artistic intent, the better. With long exposures though, you have to be careful about things that you don't generally percieve as moving objects moving (trees, stars, grass).