Hi,
The dimensions, weights, shape etc of Bo Shuriken change from Ryu to Ryu, however most have similar "effective" ranges, which are typically less than Bruno has suggested.
We'll use as an example a single pointed shuriken, a la Kukishin Ryu or Katori Shinto Ryu, with the weight spread evenly throughout the length of the spike. If you are within about 6 feet (2 metres), then you have the point aiming up in your hand as you throw it, and it'll go straight to the target, point first (without spinning). Move out to 12 feet (about 3-4 metres), and it's generally a good idea to turn the blade around so that the point aims towards the base of your hand. When thrown, the blade will do a "half spin", turning to aim the point at the target again for impact. To go further out (18 feet - up to 6 metres), turn the Shuriken again to have it tip up, and the throw will turn it 360 degrees to strike the target again. Any more than that, and there's too much risk of not impacting with the point. You can help this by chosing a double bladed version, such as the Ikko Ryu versions, or ones with larger points, making them tip heavy, and helping to ensure it always points forwards (without spinning) as it flys, such as the Negishi Ryu Shuriken.
For longer range, use Shaken, as no matter how much they spin, they will always end with a point forwards. However I'd still keep to within about 8, maybe 10 metres myself. Not only because they get harder to aim with the further they fly (actual Shaken, particularly ones like Togakure Ryu's Senban Shuriken, are incredibly light, making them easy to carry, and throw, but they can get "taken" by the wind, and move off target the longer you need them to go through the air. The major exception are the earlier tsubute variant, essentially one step past throwing a rock at someone. But their weight means that you need to be closer to be effective, anyway), but also because the longer they are in the air, the easier they are to see/avoid. And as these weapons are ideally a distraction, the element of surprise is essential, and that means employing them from a closer distance.
Oh, and for your interest, here is a link to designs of various Bo Shuriken from a range of schools:
http://www.secrets-of-shuriken.com.au/design.htm