Honestly, there is not one universal number because the lifespan depends heavily on where the bench is installed and how it is treated. In a typical urban setting, a properly applied powder-coated finish should last 10 to 15 years before you see meaningful chipping. In harsh conditions such as coastal salt air, heavy pollution, frequent impacts from bikes or maintenance equipment, that realistic window often drops to 3 to 7 years.
Powder coating is not ordinary paint. It is a dry powder that is electrostatically applied and oven-cured into a hard, continuous skin. That skin is tough against UV fading, corrosion, and light scratches, but it is also somewhat brittle. Chipping is usually caused by impact or abrasion rather than simple aging. Once a sharp knock breaks the edge, moisture and road salt can creep underneath and lift the coating further. That is why chips often spread from the ends of slats, bolt holes, or armrest joints first.
The biggest variables are surface preparation, coating thickness and chemistry, environment, and actual use. Benches that are sandblasted or chemically pretreated before coating hold their finish much longer. A 60 to 100 micron polyester or polyurethane powder coat is far more chip-resistant than a thin decorative coat. Inland parks are forgiving, while coastal or de-icing-salt environments are not. A bench in a quiet garden will also outlast one at a transit stop, skate spot, or nightlife area.
If you are buying or specifying an urban bench, ask whether the metal is galvanized or pretreated before powder coating. That combination can easily deliver 15 or more years before chips appear. For existing benches, small chips can be touched up with a matching enamel or cold-galvanizing spray to stop rust from spreading. Avoid aggressive pressure washing, and keep drainage holes clear so water cannot pool inside the frame.
So a realistic answer is this: expect 10 to 15 years before chipping in average urban conditions, 3 to 7 years in aggressive conditions, and 15 to 20 years or more for a premium galvanized-plus-powder system that is well maintained. It rarely fails all over at once. It chips at edges and high-contact points first, and how quickly you repair those spots determines how long the rest of the finish survives.