Tim, that varies a lot depending on the scene and my objectives. On most waterfalls, I tend to shoot faster than most people - typically in the 1/10-1/4s range - because I like to preserve a lot of the spray details and minimize leaf movement if it's windy. I'm generally not a fan of the "silky water" look where you just throw on a very strong neutral density filter and blur all the details into oblivion. There's no skill in that and I think it's become too much of a cliché. The exception is when I'm trying to emphasize a circular eddy or bubble trail, and in those cases the shutter speed might be anywhere from several seconds to several minutes. I always experiment with a range of shutter speeds and pick the winner later, because waterfalls take many forms and even the same one can look very different with variations in water volume. Generally I'm looking to find a speed that's just long enough to smooth the flow so the water doesn't look what I'd call "chunky" or "choppy", and that depends mainly on how far the water drops unobstructed.