This photograph:

was, I believe, about a 45 second exposure. The length of exposure was driven by a few things: the aperture size (small, to get good DOF), and the low light level.

There are quite a few things that need to be taken into account, as mentioned above. Wind, DOF, speed of the water, etc. all need to be taken into account. Your best bet is to experiment: find a stream (or something similar), and make images using different exposure times. Try and remember how fast the faster was moving; once you have the negs in hand, you'll be able to draw your own conclusions about what shutter speed will give you the 'look' your after

Good luck!