What is the life expectancy for the 6 ft. Corrugated Galvanized Steel 29-Gauge Roof Panel?

They should last several decades.

If properly taken care of the Metal Panels should be there for several decades. A couple of things to keep in consideration, if you are within 5 miles of the ocean, the salt spray will have a corrosive effect on the galvanized coating lessening the lifetime of the metal panels. The metal panels should not come in direct contact with the ground (when wet, the panels require adequate ventilation to dry out to prevent oxidation). Panels should not be installed with pressure treated lumber, the galvanization reacts to the chemicals used in treating the lumber and can cause premature oxidation leading to panel deterioration.