The exposure settings to stop moving objects depends entirely on how fast the objects are traversing your frame. Falling snow falls at a variable speed depending on the particular storm. The focal length of your lens will determine the size of your frame, so the answer is different for a 100mm lens and a 35mm lens. Another factor is how far away the exposed objects are from you.
One technique would be to measure the time it takes for an object to traverse the frame, and divide that by the relative size of the object to the frame. So if it takes 2 seconds for a snow flake to travel from the top to the bottom of the frame, and the snow flake appears to be 1/100th the height of the frame, you would arrive at 1/50th of a second. I would double or quadruple this, so you get 1/100-1/200.
But most likely you will be fighting the other aspects of the exposure. I would crank up the ISO to the highest setting that is acceptable, open the aperture for the composition that you want, and see what the meter tells you.
Depending upon the composition, you can also cheat by following the falling snow with the camera during the exposure. This will blur the background, but if you are just looking to stop falling snowflakes and you don't have enough exposure breathing room, this might work.
If you posted some examples of prior failed experiments, folks here might be able to spot where the exposure went wrong.
Another thing to consider, snow is hell on a camera's brain. White balance and exposure computations are nearly always off with snow. If you can shoot in RAW, you can do the white balance yourself after the fact. The best way to deal with metering miscomputations is to carefully review the results on the spot and adjust as needed.