Honestly, there is no single number, because it depends on the metal and the finish. But if you mean a typical powder-coated steel or cast-iron bench left outside in a rainy climate, expect the first visible rust at scratches, bolts, or water-collecting joints within about 1 to 3 years. With a high-quality coating and basic care, the frame may last 5 to 8 years before rust becomes really obvious. Thin or cheap steel benches can show rust in 6 to 12 months.
Rain alone does not instantly destroy metal. What causes problems is trapped moisture, scratches through the paint, and standing water. Once the coating is breached, oxygen and water reach the steel or iron, and rust starts. In a humid or rainy climate, that process speeds up because the metal rarely dries out completely.
Material makes a big difference. Powder-coated steel is common and affordable, but any chip or scratch turns into a rust spot quickly. Cast iron is thicker and can look rusty on the surface for years before it loses structural strength. Wrought iron sits somewhere in the middle. Aluminum does not rust the way steel does, though it can develop white oxidation. Stainless steel, especially marine-grade 316, resists rust much longer, but it can still show tea staining in coastal or polluted rain.
If you want to stretch the life of an outdoor bench in a rainy area, focus on the details. Keep the feet from sitting in wet soil or puddles. Tighten hardware, and replace cheap steel bolts with stainless or coated ones. Clean leaves and dirt off the seat because they hold moisture. Touch up scratches immediately with rust-inhibiting paint. Apply a car wax or metal protectant once or twice a year. If possible, move the bench under cover during the worst rainy season.
So a realistic answer is this: a typical metal bench in a rainy climate will usually start to show rust in about 2 to 4 years if it is not maintained. With good coating and small repairs, it can last 7 to 10 years or more before the rust becomes structural or too ugly to ignore.