Air conditioners are built to be sturdy and to stand up to all kinds of weather year after year. A typical central air conditioner should last you 8-12 years or more with yearly air conditioner maintenance from. Covers actually trap moisture inside your air conditioner for long periods of time which could then lead to the rust as well. Putting covers over the unit also provide a tempting spot for small animals to nest during winter months. Manufacturers also do not mention in their manuals about covering the units.