Honestly, there is no single answer because it depends so much on the material, the climate, and how much abuse the bench takes. But if you are looking at a typical city park bench that gets full sun, rain, snow, and maybe a few skateboarders, here is the realistic breakdown.
For wooden benches, the lifespan before serious rot sets in is usually around 10 to 15 years for common softwoods like pine or fir, even if they are pressure-treated. Untreated pine might start showing soft, spongy rot in as little as 3 to 5 years in a humid climate. Hardwoods like teak, ipe, or oak are a different story. Teak and ipe can easily last 20 to 30 years before rot becomes a structural problem, partly because of their natural oils. Oak falls somewhere in between, maybe 15 to 25 years depending on whether it is white oak (very rot-resistant) or red oak (much less so). But rot is sneaky. It usually starts where the wood touches metal brackets or where water pools in joints or end grain. You might see surface weathering and gray discoloration within a year, but that is not rot yet—that is just UV and moisture damage.
Metal benches are trickier because "metal" covers everything from cheap powder-coated steel to marine-grade stainless steel. Bare steel will rust visibly within months if the paint gets scratched, and deep rust can compromise the structure in 5 to 10 years in a coastal or de-icing salt environment. Powder-coated steel benches, if the coating stays intact, can easily last 15 to 20 years before rust breaks through at welds or bolt holes. Galvanized steel adds another 10 years on top of that, so figure 25 to 30 years. Aluminum does not rust in the traditional sense, but it does corrode and pit, especially near saltwater. A quality aluminum bench can last 30 to 40 years with only cosmetic surface oxidation. Wrought iron is beautiful but high-maintenance; without regular repainting, it can rust through in 10 to 15 years. Stainless steel is the champion—40 to 50 years or more, but it is expensive and rarely used for public benches except in coastal areas.
The real killer for both wood and metal is trapped moisture. A bench with good design—sloped seats, drainage gaps, stainless steel fasteners, and no horizontal pockets where water sits—will outlast a poorly designed bench by a factor of two or three. So if you are buying or maintaining an urban bench, think less about the material label and more about how water flows off it. And remember, a quick coat of sealant on wood or touch-up paint on metal every couple of years can double the service life. That is the difference between a bench that looks wrecked in five years and one that still looks decent after twenty.