Galvalume Roof?

Have a house which sits approx 75 ft from a salt water bay. The bay is sheltered from the Gulf of Mexico by a key.
What is a realistic expectation of roof life (baring a hurricane) using 5-V 26 gauge galvalume with mill finish?
The contractor advises that they have seen galvalume roofs under same conditions still look good after 15 years. That the only issues he's had to deal with (with that type of roof) involved re-calking.
Ted Borck

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