I find that the minimum exposures for star trails is 20-25 minutes. The best results are from 40 minutes and above. But in that case, you must check that everything is ok before you set it on bulb.
I personaly have NR on, since if I do star trails it will probably be just one shot anyway, so I do take some time to find the best place, which I will probably know from before, take some time to compose correctly, use hyperfocal distances with manual focusing to check if everything is sharp to infinity, decide my exposure times, set the appropriate aperture and push the button on my remote shutter. I then sit in the car, read a book, I once took my laptop with me and watched an episode of House MD. I knew that when the episode finished it was time to close the shutter

. Until I drive home the image would have already be recorded on the CF, noise free.