Yes, an 8-stop reduction means you are reducing the incoming light by a factor of 256. If your camera metered a correct exposure at 1/1000th of a second without a filter, adding an filter would require a shutter speed of 4 seconds (1/1000 -> 1/500 -> 1/250 -> 1/125 -> 1/60 -> 1/30 -> 1/15 -> 1/8 -> 1/4) to achieve the same exposure.
This article will explore the science, the creative applications, and the specific gear you need to master without sacrificing image quality. exposure x8