Per the manufacturer, the active ingredient in Eco-Mate ARMICARB 100 Fungicide
is active no matter how long the bag has been open. If moisture gets in and hardens the material, all you will need to do is crush it back up, and it will be fine. They did not indicate a shelf life for this product. If you have any additional questions, feel free to contact them directly at 901-761-0050 Helena Chemical.