Honestly, there is no single universal answer, but for most wooden urban benches, plan on resealing once a year. In harsher settings such as coastal air, heavy rain, snow, intense sun, or constant public use, you may need to do it every six to twelve months. If the bench sits in a sheltered spot and gets light use, a high-quality penetrating sealer can sometimes last up to two years.
The real answer comes from the bench itself. Do a quick water-bead test: sprinkle a few drops of water on the wood. If the water beads up, the sealant is still working. If it soaks in and darkens the wood within a few minutes, it is time to clean and reseal.
I usually recommend checking twice a year: once in spring after winter, and once in early autumn before the rainy season. Urban benches deal with more than weather. Sun, pollution, bird droppings, food spills, and even pressure washing can break down the finish faster than a backyard bench.
Use an exterior-grade penetrating oil or wood sealer made for hardwoods or dense softwoods. Avoid thick film-forming varnishes on slatted benches unless you enjoy sanding and peeling. Clean the bench first with a wood cleaner or mild soap, let it dry fully, lightly sand if the surface is rough, then apply thin coats with a brush or rag. Two thin coats are better than one thick coat.
If you manage a public bench or a bench in a high-traffic urban area, schedule maintenance once a year as a baseline, and let the water test tell you if you need an extra coat sooner.