As per the technology in the backpacking water filters, these won’t get destroyed if kept unused for long. But, you need to store it well and maintain it properly. While storing, you need to dry it well and then store, and else it will catch mold. The lifetime of a water filter depends on the number of gallons it pumps. If it has not pumped that much water, you can use it. If you find the filter gets mold and not suitable for further use, you can replace the cartridge and use it along.