A piece of properly-./#Maintained cast iron cookware will last generations.
Maintain
Cast iron maintenance just consists of keeping it ./#Clean and ./#Seasoned.
Clean
After cooking, serving, and eating, scrape any food off and hand-wash the cookware as you would any other pan. Wipe it dry immediately after washing (do not let it air dry, do not wash in the dishwasher) and, if you want, apply oil the same way you would when ./#Seasoning it. This last step is a good idea especially if you only rarely cook using your cast iron cookware, but once you have a good layer of seasoning built up it becomes less necessary.
NOTE: Do not use lye-based soaps on cast iron cookware. This will strip the seasoning and will require you to re-season your cookware. You might see advice to never use dish soap on your cast iron cookware but this advice comes from a time when dish soap was made with lye and was therefore much more highly concentrated than the dish soaps we have today. Most modern dish soaps should not include lye unless they are particularly strong, and they should be safe to use on cast iron cookware.
Season
Cast iron seasoning is a layer of oil baked onto cast iron and carbonised through polymerisation. That carbonised oil is what gives your cast iron that black patina. Seasoning forms a natural, easy-release cooking surface and helps prevent your cookware from rusting.
The easiest way to keep cast iron seasoned is simply to cook with it; every time you cook with oil, you're potentially adding another layer. Explicitly adding another light layer of ./#Oil after ./#Cleaning may also be a good idea if the cookware is new.
The second way is to season it directly, and this should really only be necessary when you've done something to remove existing seasoning. That can happen when you cook acidic foods, use excessive heat, scrub with abrasive utensils or scouring pads, or ./#Clean it with particularly strong lye-based dish soap rather than regular dish soap.
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Drip one or two drops of ./#Oil inside the cookware and spread it around, both inside and outside, with a paper towel
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When finished spreading, the cookware should have a slight sheen, but not look wet. If it looks wet, you used too much oil. As long as the oil doesn't run and collect when you angle the pan, don't bother trying to get the excess off because it won't hurt anything.
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Pre-heat the oven to 500° F (260° C)
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Once preheated, place cookware upside-down on middle shelf
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Place baking sheet on bottom
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Bake for one hour
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Open door and let cookware cool for a while before handling it
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Wipe cookware down with paper towel
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The paper towel may or may not come away slightly or really black. This is normal; coming away mostly clean (aside from the oil) is a sign that the cookware's been well-used and cared-for. If it comes away mostly black, it's just a sign it might have been bought recently.
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Oil
Different oils have different flavour profiles. They also smoke at different temperatures. Note that all cooking oils and fats can be used to season cast iron. Available, affordability, effectiveness, and smoke point were all evaluated when creating the chart below.
The values in this table represent the typical smoke point for commercially available edible oils. Smoke points may vary within a source oil due to such factors as processing techniques and/or seasonable variations.