Question about stong ND's

With ND's, like 10 stops, for extremely long exposures, when you turn it to bulb, how do you know how long you wait for the exposure to go?

Start with an exposure that the camera can figure out for you. If it says 2 second exposure then you add the ND increase by 10 stops and use a stop watch or something with a second hand. Often times you may have to adjust it a little based on the histogram. Expect to take 2 or 3 shots to get it dialed in.

A 10 stop filter lets in 1000 times less light, so work out your exposure without the filter, then multiply by 1000. So if the shutter speed is normally 1/200, set it to 5 seconds with filter on. 1/30 becomes 33 seconds and so on.