Honestly, with a good gable vent, it should never need replaced. But it really depends on the material it is made of for how long it might last.
How often should a vinyl gable vent be replaced? With vinyl or plastic gable vents, the replacement rate is high due to the vent not really being made to last. Usually these are pretty cheap for this same reason. Most vinyl vents will last about 2 to 10 years before one hazard or another will finally strike. Squirrels like the ease of chewing through the soft material to enter a safe dry attic usually within the first 2 years. If they don't, then, the louvers sagging from overheating is the next obstacle. By some chance, they allow enough ventilation, then the next is dry and brittle slats that will eventually snap and fall out. Vinyl vents are advertised as a lifetime vent. Most need replaced within the first 10 years.
How often should a wood gable vent be replaced? Wooden gable vents do last a little longer. If properly painted and sealed, they can last a lifetime of 20 years or more. These do require a lot of maintenance for them to last. Usually, painted every 2 years. Squirrels do like chewing wood and can enter quite easily. Nature's only hazard to these is rot. But if rot starts, it will continue even if it is painted and sealed correctly. Wood putty and caulks only trap water as it releases from the wooden surface. Once the caulk begins turning loose or the wood putty starts gapping, there will be a minute slit for water to begin entering again. The raised edges of these will act as a dam preventing the water to run off. Although repairing a wooden vent may give another year or two, it is not a permanent solution.
How often should galvanized gable vents be replaced? Hopefully never but the do rust out if they are not painted good. And sometimes that is still not enough. The vibrations from high winds will cause the louver ends to scrub and remove the paint to start rusting. The single pop rivet design and fastened in a track makes it easy for louver movement. This vibrating motion also causes the pop rivets to loosen and eventually release the louvers to fall out. The best preventative maintenance for gable vents to extend the gable vent's life is to inspect them visually once every year or two either in the spring or fall.
How often should aluminum gable vents be replaced. Never! Aluminum gable vents manufactured properly will truly last a lifetime. Aluminum will not rot or rust and is not affected by attic heat. Poorly made vents of this metal can be spotted by spotting visual pop rivets or tack welds. This commonly means the louvers are locked in a track system and the fasteners will be weakened with age from wind vibrations. Or if it is an adjustable pitch triangle vent, the slats will be weak in the middle and often sag from their own weight as time goes by. Getting a quality aluminum vent will last a lifetime though.