The short answer is: it depends on the bench. A high-quality street bench made from UV-stabilized materials can sit in direct sun all day for years with very little noticeable fading. A cheap bench made from untreated wood, basic plastic, or low-grade paint will start to look chalky, dull, or bleached within a season or two.
The real issue is not sunlight by itself. It is ultraviolet radiation. UV rays break down the chemical bonds in paint, dyes, plastics, and wood fibers. Heat makes that breakdown faster. So a bench in Arizona or southern Spain will usually fade faster than the same bench in a cooler, cloudier climate, even if both get the same hours of daylight.
Material matters more than anything else. Powder-coated steel or aluminum is one of the best choices if the coating is UV-stable. The color holds well, but deep scratches can let moisture in and should be touched up. Recycled plastic lumber, especially HDPE, is another strong option because UV inhibitors are mixed through the entire board rather than just painted on the surface. It can handle full sun very well, though the color may shift very slightly over many years. Thermoplastic-coated metal benches are also tough and fade-resistant.
If you like the look of wood, teak and other dense hardwoods do not fade the way painted surfaces do. Instead, they naturally weather from warm brown to a silvery gray patina. That change is normal and does not harm the wood if the bench is cleaned and occasionally oiled, sealed, or protected. Softwoods like pine or low-quality plywood are the worst choice for a sunny spot because they can fade, warp, crack, and weaken quickly. Cheap plastic furniture without UV stabilizers can become brittle and chalky in direct sun.
So if you are choosing a street bench for a permanently sunny location, look for these terms: UV-stabilized, UV-resistant, powder-coated, marine-grade, or HDPE lumber. A good supplier should be able to tell you whether the color is rated for constant sun exposure and whether fading is covered under warranty.
Maintenance also helps. Wash the bench with mild soap and water a few times a year to remove dirt and pollution that can make fading look worse. Keep metal scratches touched up so rust does not spread under the finish. If the bench is wood, use a UV-protectant oil or sealer made for outdoor furniture. For plastic lumber, a simple wash is usually enough. Avoid harsh solvents or abrasive cleaners because they can damage the protective surface.
Placement still matters. If you have any control over the spot, a bench under a tree, canopy, building shadow, or pergola will last longer and look better. But if the bench must sit in full sun all day, do not assume every bench can handle it. Choose one that was built for that exact job.
So yes, a street bench can be left out in the sun all day without significant fading, but only if the material and finish are designed for UV exposure. If you buy a cheap bench and park it in full sun, fading is not a question of if. It is a question of how soon.