My family and I are in the process of getting our house ready for sale. The realtor suggested we clear out as much stuff as possible and place them into storage. I store my supplies in plastic tubs that I lined with cedar. I will be placing these tubs in boxes that will hold several. I thought I would add moth balls or crystals to the boxes for added protection in storage. Can someone tell me how often I should replace or replenish to keep them effective?