it really depends on how bright the scene is.
Hard to say.
You need to know how slow a shutter speed to make the water flowy, then work backwards from there.
I think if in strong daylight (eg. mid-day), you'll need a VERY dark ND filter
my point is that just because someone achieved a desirable effect with a particular piece of equipment, doesn't mean that using the same equipment will get you the same effect.
This is quite naive thinking. Photography is much more about light (angle, intensity, colour, etc) than it is about equipment.
I agree that there is no 1 ND filter that can suit all situations. Like night86mare mentioned, it's good to have various ND filters and adjust accordingly. Alternatively, you can opt for the fader ND which the stops can be varied accordingly.