Typically in a home with a crawl space, the ground around the house is higher than the earth under the house. Water will seek the lowest point, which is most likely under your house. I agree with Kevin that the proper corse of action is to improve the exteroir drainage with drain pipe and proper grading. However, that's usually an expensive fix. Since you are not going to eliminate the source of moisture, all that can be done is to RETARD it (slow it down). This is what the 8 mil poly under the house will do. It will slow the transmission of moisture to vapor. The small amount that gets past the plastic can be carried off by the foundation vents. Therefore, DO NOT cover your foundation vents. That would cause a build up of moisture. After installing the plastic, a moisture meter is needed to monitor the drying of the wood. In can take anywhere from a few days to a few weeks. On average, under normal, dry conditions, about one week. But this isn't about how long as it is how much. That is why a moisture meter is needed. Without it, there is no way to tell IF the conditions are acceptable.