The honest answer is every one to three years for most wooden street benches. If your bench sits in full sun, gets rained on, or faces a street with splash from cars, I would lean closer to every 12–24 months. If it lives in a sheltered spot and you use a high-quality stain, you can sometimes stretch that to three years.
The type of stain matters more than the calendar. Semi-transparent stains and penetrating oils look natural but have less pigment, so they usually need reapplication every one to two years. Solid stains and heavier exterior finishes last longer, often two to three years, but they can peel if moisture gets trapped. A clear sealer alone is not really a stain. It will not protect much against UV, so if you use one, expect to refresh it at least once a year.
Signs it is time to restain include water no longer beading up, color looking dull or gray, wood feeling rough or splintery, and dark spots or mildew showing up. If the seat and arms look dry or thirsty, that is also a strong signal.
A wooden street bench is in a tough spot. It gets full sun, rain, snow, ice, and sometimes road salt or car exhaust. UV rays break down the stain, while moisture causes swelling and shrinking. If you live somewhere with harsh winters or strong summer sun, annual maintenance is safer. In mild climates, every two years may be enough.
To make the stain last longer, clean the bench first with mild soap and water or a wood cleaner, then let it dry completely for at least 24–48 hours. Sand lightly if the surface is rough, using 80–120 grit sandpaper, and wipe off all dust. Stain on a dry day when the temperature is between about 50°F and 80°F, and avoid direct hot sun because the stain can dry too fast and look blotchy. Apply thin coats rather than one thick coat, and wipe off excess with a clean cloth. Pay special attention to joints and end grain because those areas soak up water first.
Between stains, sweep off leaves and dirt regularly, wash the bench with mild soap and water once or twice a year, tighten loose screws or bolts, and touch up worn areas before they spread.
If you want to keep a street bench looking good with minimal guesswork, plan on staining it every year or two. Use the way water behaves as your guide. Once water stops beading and starts soaking in, it is time. A little maintenance at the right time will keep the wood from turning gray and cracked, and it will save you from a much bigger sanding job later.