Honestly, it depends a lot on what the furniture is made of and where it sits. A good commercial-grade bench, planter, or café table designed for urban outdoor use can handle being outside all year and still look decent after 10 to 20 years—sometimes longer—if it gets basic care. A cheap residential-looking piece might start fading, wobbling, or rusting within a year or two.
All-year exposure is not just rain. It is UV, freeze-thaw cycles, pollution, bird droppings, salt from roads or sea air, constant foot traffic, and sometimes vandalism. That is why material and construction matter more than a nice finish.
Powder-coated aluminum frames are a strong choice for city furniture because they resist rust and are light enough to move. Stainless steel is excellent, especially marine-grade 316 in coastal areas, but even stainless needs cleaning to prevent pitting. Teak and other good hardwoods can last for decades, but they weather to gray and need occasional oiling if you want a uniform look. HDPE and high-quality synthetic wicker are very practical: no rotting, low maintenance, and good UV resistance. Concrete and stone are extremely durable but heavy. Mild steel or thin tubular frames with poor coating are the weak link—once water gets in, rust follows.
Hardware and drainage often decide how long it really lasts. Stainless screws, welded joints, drainage holes in seats and planters, and breathable covers make a big difference. Cushions and slings are usually the first things to fail, so removable, quick-dry cushions that can be stored during bad weather are smart.
Maintenance is modest but not zero. Rinse off dirt and bird droppings, check for chips in powder coating, tighten bolts, clean drains, and touch up scratches before rust spreads. In snowy or coastal cities, a quick seasonal clean goes a long way.
So, my short answer: quality urban outdoor furniture left outside all year is quite durable—often 10–20+ years—but only if it is specified for the climate, well built, and given occasional care. Cheap furniture left outside all year? Usually not durable at all.