Try some exposures of 1, 10, 30 and 60 seconds with NR turned on and off and compare the results. I suspect you will be easily ably to see the difference.
On my Canon 40D the default setting is that the camera will perform NR on exposures of longer than 1 second. This is a good indication that the designers of the camera perceive that noise may increase to become objectionable at exposures of longer than 1 second.
Cheers
Dennis
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