Are tin roofs "permanent"?
I guess you mean a galvanized or otherwise coated steel. Life depends on weather, specifically rain. In a desert it will last forever. If the current coating is repaired at any damage spots, it will last almost forever. "Repair" could be anything from dabbing on roofing tar to sandblasting and coating with a zinc-rich epoxy (typically used to protect industrial steel surfaces). Even conventional paints could be used - paint on cars and houses lasts pretty well. It depends on how you want it to look and how often you are willing to "touch it up". For industrial steel tank coatings (aka oil tank farms), touch-up is part of the maintenance program. The contractor is just trying to make money. Addition : There are also "aluminum" coatings commonly sold for mobile home roofs. These may sort of match your galvanized in appearance.