There is no limit as far as a regulatory requirement. What it comes down to is whether the airplane passes an annual inspection. We look real hard at corrosion and cracking. The airframes of airplanes that are kept in hangars tend to be quite free of corrosion problems. I always look real hard at that when I pull out all the inspection panels. As far as engines and other items are concerned, those are inspectied and replaced or overhauled as needed.
