Short answer: it depends on the material and how much maintenance it gets. A good outdoor bench that’s rained on all year can last anywhere from about 5 to 25+ years, but cheap wood or unsealed metal can fail much sooner if it never fully dries out.
Teak is the gold standard. Genuine teak can easily survive 20 to 30 years outside in the rain, even with very little care. It slowly turns a silver-gray color, but the natural oils keep it rot-resistant. Cedar and redwood are also solid wet-weather choices and typically last 10 to 20 years. Pressure-treated pine can last 8 to 12 years if it’s sealed or painted regularly, but raw pine sitting in constant moisture may start rotting in as little as 3 to 5 years.
Metal is trickier. Aluminum and powder-coated steel frames usually hold up for 10 to 20 years in rain, especially if the coating stays intact. Wrought iron looks great but will rust once the paint chips, so you often get 5 to 10 years before it needs sanding and repainting. Cheap steel can rust through in just 2 to 4 years.
Plastic, poly lumber, and composite benches are the most rain-proof. High-density polyethylene or poly lumber can last 20 to 30 years with almost no maintenance. These won’t rot, crack, or rust and just need an occasional wash.
If you want a bench that can truly be left out all year with minimal fuss, choose teak, aluminum, or poly lumber. No matter what you buy, keep the feet out of standing water and tighten bolts once a year to add extra years to its life.