An ND 10 will allow a sutter speed 10 stops slower than the one you would use without it......
if you are shooting at f8 and normally you would get 1/250 to match it in the light available. Then with the ND10 you would extend your shutter speed to 4 seconds.
I am sure you can work out the rest - f11 would mean 8 seconds, in the above example.......how long do you want? You might need to slow your film speed to get a bit more.
possibly up to 10 - 15 seconds..... Is there a lower ND i could manage with for these times? inc adjusting mmy camera settings etc