I’d put the realistic range for a decent powder-coated steel urban bench in a normal inland climate at 15 to 25 years. Away from salt, the main threats are UV fading, rain, freeze-thaw cycles, bird droppings, and occasional de-icing salts. With a galvanized base under the powder coat, many inland benches reach 20 to 30 years before the frame needs major work; aluminum and 304 stainless can go even longer structurally, though their finishes may dull or get minor surface staining.
On the coast, the same bench lives a much shorter life. Salt spray, humid marine air, and wind-driven chlorides attack metal constantly. A standard carbon steel bench with only a basic coating may show rust bleeding, coating blisters, and fastener corrosion within 3 to 7 years. A good galvanized and powder-coated bench might give you 8 to 15 years with regular washing and touch-ups. If the spec is upgraded to 316 stainless steel or heavily coated marine-grade aluminum, you can realistically expect 20 to 30+ years, but maintenance still matters because salt sits in joints, under seat slats, and around bolt holes.
So if you’re comparing inland versus coastal, a simple answer is: inland 15–25 years, coastal 8–12 years for typical coated steel. But material selection changes the picture more than location alone. For coastal installations, I’d avoid uncoated carbon steel, choose 316 stainless or powder-coated aluminum, isolate dissimilar metals, use stainless hardware, and schedule rinsing at least a few times a year. Inland, you can often get away with galvanized or well-coated steel and basic inspections, but touch up scratches early to prevent rust from spreading.