Bulb mode allows you to control when the exposure starts and finishes, which means that you control the shutter-speed. Together with ISO and Aperture, you still need to balance the 3 parameters and get a proper exposure. See Exposure Triangle if you are not familiar with the concept.

To be clear, you do not reduce the shutter-speed to bulb. You take control of shutter-speed with bulb.

Now you have to use choose a shutter-speed. The easiest way is to get the camera to meter and then balance with other exposure parameters. Usually, I take a shot in Aperture priority mode at a ridiculous ISO, say 25600. If it comes out exposed properly, note the shutter-speed and then calculate how long it would take at a low ISO, say 200. You can work this out in your head but you don't have to anymore, as they say there's an app for that. The one I use simply called Exposure Calculator.

The other option is to meter at the desired ISO with a set aperture again and note the shutter-speed. Then, add an ND filter, do the math to adjust the shutter-speed. Again, there is an app for that. The one I use is called ND Filter Calculator. For ND filters, the math is very simple. ND4 == 2 ^ 4 == 16, so multiply the shutter-speed without the ND filter by 16.

Some cameras show a counter on the screen or on the top LCD. If that is not the case with the EOS M, use a watch which shows seconds. You should really not have to worry about fractions since it is hard to react with accuracy.