Although it depends on the weather and humidity, you should only apply flooring polyurethane when you are certain the earlier coat is totally dry. In some dry climates, this might be as little as 8 hours, while in humid areas, it might mean as much as 2 days. To determine whether your floor is ready for a second coat, check the polyurethane itself. If it still feels tacky or still attracts sticky dust, then it's too early for another coat, and if there is still too much dust, you may have to strip the new polyurethane and start the process again. Generally, however, if you can buff it to a shine, you can put down a new coat.