Fabric covers (also called membranes) useful life is between 15 to 20 years depending on the quality of raw materials used in their fabrication, weather conditions, climate, exposure to UV and physical damage. When aging, fabric covers will weaken, get thinner and will be most likely exposed to tears and loosening. Small tears and rips can be repaired depending on size and location but a worn cover should be replaced.
Prior to 2010, fabric membranes were part of the structural integrity of the building. Therefore, it's highly recommended to inspect the cover on a regular basis to ensure it's still in good condition. Regular inspections of the overall cover's condition, tension, ratchets, structural components will provide you peace of mind. After all, you need to protect the material stored underneath the structure and more importantly, people working in and around you fabric building.