The radiator cap on your vehicle should last you the duration of the life of the vehicle, unless it is subjected to heavy rust. Check your radiator cap at regular intervals between six and twelve months.
Weather conditions play a large part in the life of your vehicle and, depending on the climate where you live, the effects on your car will vary. Coastal climates are very susceptible to salty conditions from the sea air and cars in such areas will be targeted by oxidation and rust. Colder areas of the country and places which receive higher than average amounts of snow are also going to be vulnerable to faster rusting and metal components will wear out faster.
Inside the Hood
There is no exact way to prevent damage under the hood. It may seem more protected than that the rest of the vehicle because it is underneath a huge metal cover but rain water, salt and other natural abrasives will still creep under there an cause rusting. Check your radiator cap every six months and make sure the spring is clean and rust free and the rubber seal is still in good condition. If you notice signs of wear then you should get a new one.