Here's a quote from the Virginia Cooperative Extension Soil Test Report I received: " The lime recommendations are for a single application that will last from two to three years. After that time the soil should be retested. For best results, lime should be applied several months ahead of the crop to be planted to allow time for a more complete soil reaction." It is kind of general, but thank goodness I don't have to worry about it. My soils ph was 7.3 :-)