Unless you are doing astronomy photography, keep long exposure nr off. It will take a dark frame of the same length as the exposure and subtract it to remove thermal noise. So, if you have a 5 minute exposure, another 5 minute dark frame exposure will occur. People turn this on and then think their camera is defective.
Actually, for astrophotography, I would turn NR off and do all of that with post. I would be afraid of losing *any* details with in-camera NR. The astrophotography-specific post processing applications do this best by averaging many fark frames.
One reason that Canon dominates DSLR astrophotography is because on Nikon DSLRs, you couldn't turn off all in-camera NR, even with RAW files. As a result, Nikon DSLRs got the nickname "star eaters" among many astrophotographers.