What causes corrosion in most instances is moisture or humidity swings. They keep cool, damp or dewy, they heated up and dry out, then go through the same cycle again. Your garage does work as the first barrier to these humidity swings, but it is obviously more susceptible to the changes than the interior of a temperature control home. If you wipe the mags down after they have acclimated to the garage, they will stay fine for a long time. If you have wide temperature and humidity swings or just high humidity like we do in Florida, then metal mags might need to be checked more often. Aluminums should be fine for a long while as well as any polymer. Heat and direct sunlight are the enemies of polymer.
Really, I would not store any mag loaded for potential use in the garage. Those stay in humidity controlled space. That does have more to do with the ammo than the feeding device. But if you have the standard blued AK mags or even blued steel AR or Mini 14 mags, I would set them in place, let them acclimate for 24 hours, wipe them all with a cloth and some preservative oil and they should only need retreating maybe once a year. They will do better in a protective container so they do not have the same humidity swings.