Honestly, there is no single answer because it really comes down to the material, the finish, and how intense the sun is where you live. That said, if a bench is in direct sunlight all day, most finishes and materials will start to show some change within a few weeks to a few months. By the end of the first full summer, the fading is usually pretty obvious.
Wood benches are the most variable. A natural cedar or teak bench may not fade the way people expect. Instead, it weathers to a silvery-gray patina. That can begin in as little as 4 to 8 weeks in strong sun and often becomes even across the surface within 6 to 12 months. If the wood is stained or painted, especially a dark color, you will likely see noticeable fading in 1 to 3 months, with more dramatic color loss after a year.
Plastic and resin benches tend to be the fastest to show sun damage. Cheap polyethylene or polypropylene can look chalky and washed out within a few months and may become brittle after a season or two. Higher-quality HDPE or poly lumber is much more UV-stable and can hold its color for years, though even it can slowly lighten over time.
Metal benches are usually fine until the coating fails. A quality powder-coated aluminum or steel bench can keep its color for 1 to 3 years or more, even in full sun. But a cheaply painted bench may start fading and chalking within one summer. The bare metal itself does not really fade, but the paint or powder coat does.
Wicker and rattan depend on whether they are natural or synthetic. Natural rattan fades and dries out fairly quickly in full sun, often within a few months. UV-resistant resin wicker is much tougher and can last several years before fading becomes obvious.
The other thing people forget is that fading is not just about color. UV rays break down the surface over time, which can lead to cracking, chalking, splintering, or brittleness. So a bench can look old even if the color has not changed dramatically.
To slow it down, use a UV-protective sealant or outdoor furniture spray on wood, choose lighter colors, keep the bench covered when not in use, or place it in a spot with afternoon shade. If you are buying a bench for a full-sun area, look for powder-coated metal, HDPE poly lumber, or UV-stabilized resin wicker. Those tend to hold up the longest.