There is no definitive answer to this question as it depends on the type of bicycle, the rider's weight, and the riding conditions. Generally speaking, spokes should be tightened to a tension that is high enough to keep the wheel from going out of true (becoming warped), but not so high that the spoke threads start to strip. A good starting point is to tighten the spokes to a tension that is about half of the maximum that the spoke nipple can handle.