Honestly, it’s one of those “how long is a piece of string” questions, but I can give you a very realistic picture. You’re asking about a metal bench left fully exposed to sun, rain, snow, and maybe even salty coastal air. The short answer is: a cheap, mass-produced bench might start looking rough and get structurally wobbly in 3 to 5 years, but a high-quality, well-made metal bench can easily last 30, 50, or even over 100 years. I’ve sat on cast iron ones in old English parks pushing a century. The massive difference comes down to the exact metal, the construction, and whether anyone ever shows it some love.
The material is everything. Cheap hollow steel tubing from a big-box store is the worst. Water gets inside through tiny pinholes in the welds and rots it from the inside out, often turning it into a safety hazard within 5 to 7 years. Solid cast iron, on the other hand, is unbelievably durable if you don’t mind a thick patina of surface rust — it can easily span several human generations because the material is so thick that it takes forever to rust through structurally. Then you’ve got wrought iron, which is more elegant and tough, but it’s thirsty for paint maintenance; neglect it and rust will eat the joints. Galvanized steel is a great middle ground, the zinc coating sacrifices itself to protect the steel, often giving you 20+ years before red rust appears, especially if you touch up any scratches. The real modern champion is heavy-gauge extruded aluminum. It will never, ever rust. It forms a stable aluminum oxide layer and just sits there, often looking almost identical at year 30 as it did on day one, developing a specific matte grey character that many architects love. Don't mix up metals though — if you bolt a steel plaque to an aluminum bench, they'll set up a galvanic cell and the aluminum gets eaten away.
The actual killer in the elements isn’t just direct rust-through; it’s joint failure. Wooden slats on a metal frame hold moisture right against the screw holes, starting decay at the weakest point. Metal-to-metal connections often fail because of galvanic corrosion between the bolt and the frame. The sun also absolutely destroys the plastic or nylon glides stuck to the feet, so the bench ends up sitting in a puddle. And don't underestimate vandalism or heavy use adding stress to already weakened metal. A bench in a peaceful garden will outlive the same model placed in a busy skateboard-prone urban plaza.
If you want to stretch the lifespan dramatically, the rule is: keep the feet out of standing water and dry off the welds. The cheapest, most effective thing a city crew can do is every couple of years, flip the bench over for an afternoon, lightly sand any bubbly rust spots on the underside, and hit them with a cold galvanizing spray or an oil-based enamel paint that matches. Pulling out the bolts and greasing them with a marine-grade anti-seize grease prevents the frozen, cracked joints that eventually wobble a bench to death. For coastal locations, I’d say don’t even consider anything except powder-coated aluminum or high-end 316 marine-grade stainless steel, because everything else will pit within months. And last thought, a lot of what seems like a deadly decay on an old cast iron bench is just a beautiful crusty lifetime story that’s purely cosmetic; underneath that protective crust, the metal often remains perfectly sound for decades.