Honestly, there is not one universal number. It depends mostly on the wood species, the climate, and how well the bench is kept away from standing water. That said, I can give you realistic ranges based on how different outdoor benches tend to behave.
A cheap bench made from untreated pine, spruce, or fir can start showing real rot in as little as one to three years outdoors, especially if the legs sit directly on wet soil or the finish is not maintained. Pressure-treated pine does better, usually lasting five to ten years before rot becomes obvious. Cedar and redwood benches often go ten to twenty years because the heartwood has natural oils and compounds that resist fungal decay. A well-made teak bench can easily go twenty to thirty years or more before the wood itself starts rotting, which is why teak is so popular for outdoor furniture even though it costs more.
The reason there is such a wide range is that rot is biological. It happens when fungi get enough moisture, oxygen, and mild temperatures. Wood generally needs a moisture content above about twenty to thirty percent for decay fungi to thrive. If a bench dries out quickly after rain, it may last for decades. If it stays damp in a shaded corner, under wet leaves, or with its legs touching the ground, rot can start much sooner.
In most benches, rot does not start on the seat surface first. It usually begins where water soaks into the end grain, especially at the bottom of the legs, where the legs meet the ground, or in joints where two pieces of wood trap moisture. Cracks in the finish or wood also let water get inside and stay there. Once you see dark spongy spots, soft areas you can press with a screwdriver, small mushrooms, or a musty smell, the rot has already taken hold.
The good news is that you can greatly extend the life of a wooden bench with simple maintenance. Keep the legs off bare soil by placing the bench on a patio, gravel, pavers, or rot-resistant feet. Sweep off leaves, dirt, and standing water regularly. Clean the wood once or twice a year with mild soap and water, and let it dry completely. Apply a penetrating oil, wood sealer, or exterior stain made for outdoor use. Avoid thick paint or film finishes that can trap moisture when they crack. If you live somewhere with freezing winters or heavy rain, cover the bench or store it in a dry place during the worst months.
So if you are trying to decide whether a wooden bench is worth buying, think about what you are getting. A $100 untreated pine bench may look nice for a summer but can start rotting within a couple of years. A cedar or pressure-treated bench is a better middle ground. Teak is the premium option if you want something that can sit outside for decades without the wood falling apart. With good placement and yearly care, even a modest wooden bench can last far longer than the typical range.