Long Exposure Noise Reduction will accomplish two things:
1) It will eliminate "amp noise", which shows up as a generally rather large light area in a very long exposure image. It is caused by electronics dissipating heat onto the sensor. Modern cameras do not have much amp noise, and you may not see any at all.
2) It should mask stuck pixels. Pixels that do not normally stick with short exposures (less than a second or so) may stick with long exposures of 30s or more.
LENR will NOT and can not reduce random shot noise, and that is the predominant noise. For this reason you only want to use it if it solves a serious and otherwise insolvable problem.
As mentioned previously, you do not want to use LENR for star trails. Or if you do, you need to get the entire trail in one very long image (not usually practical because...)
The main limiting factor for exposure length is usually light pollution. Where I live I can only get about a 30s exposure even with wide apertures and low ISO's. If you are traveling to a dark sky site then you can get longer exposures. You have to test your sky with trial exposures.
Very long exposures can be ruined by airplanes or even satellites. If you shoot short 30s exposures you can erase any satellite trails without leaving obvious gaps where you erased (other frames will tend to fill it in). That is probably better than leaving out frames with unwanted airplane trails.
The D800 has a built in intervalometer, making it easy to shoot a relatively uninterrupted series although the exposures are limited to 30s max. You do need to set the "interval between shots" to a value about 1s longer than your exposure time.
Otherwise if you set the interval short (to 1s, as would be the intuitively obvious value) the camera is actually trying to take a shot every second while the shutter is open. The result is that the number of shots will click down 30 shots during each 30s exposure, limiting you to 33 exposures without restarting it.
You want to test this ahead of time so you understand the intervalometer settings. A 30s exposure could require an interval of 32s, for example, and it varies a bit among camera models.
If you get serious about this, Nikon makes the MC-36 remote intervalometer that I believe can shoot longer than 30s exposures. It also has the same quirk in setting the interval time (beginning of shot to beginning of next shot, not the time between the end of one shot and the start of the next).
Try using manual focus with LiveView (tripod mode) to focus on a very bright star. I've had good luck using AF, though, on bright stars. AF works better with longer focal lengths and may not work well with a wide angle lens. LiveView - zoomed in 100% - will probably give the most consistent focus, especially at wide angles.