From my experience, there isn’t one universal number, but a well-built metal outdoor bench in a public park will generally last 10 to 25 years. The exact lifespan depends on the metal, the finish, and how much abuse it takes from weather and visitors.
A standard powder-coated steel bench is the most common park option. In a temperate climate with regular cleaning, I’d expect about 10 to 15 years before the coating starts to break down and rust shows up at scratched or chipped areas. In coastal or high-humidity environments, that can drop to 8 to 12 years unless you wash it and touch up the coating.
Galvanized steel benches do better because the zinc layer protects the steel even when the surface is scratched. Those often last 15 to 25 years, sometimes longer if the galvanizing is thick and the fasteners are stainless.
Aluminum benches can easily last 20 to 30 years or more. Aluminum doesn’t rust like steel, though it may develop a dull oxide layer or minor pitting. Light sanding and repainting can keep an aluminum bench looking decent for decades.
Cast iron benches are a bit different. Structurally they can survive 30 to 50 years or more, but they need repainting and rust treatment every few years. If they’re neglected, the legs and joints can corrode badly even though the metal is thick.
Stainless steel is the longest-lasting metal option, often 25 years plus, but it’s not common in parks because of the cost.
In practice, a public park bench takes a beating from sun, rain, snow, temperature swings, salt, bird droppings, scratches, and occasional vandalism. The lifespan isn’t only about the metal itself. It’s about the coating system, drainage, hardware, and whether the park has a maintenance schedule. A bench that gets inspected, cleaned, and repainted every few years will often last twice as long as one that’s left alone.
So if you’re asking for a realistic planning number, I’d use 15 to 20 years for a standard powder-coated metal bench in public use. If you upgrade to galvanized or aluminum and maintain it properly, 20 to 30 years is very achievable.