The honest answer is: it depends. There's no single universal schedule, as the frequency hinges on several key factors. However, a good general rule is to inspect and likely re-treat your wooden outdoor bench every 1 to 2 years. For harsh climates with intense sun, heavy rain, or snow, annual treatment is often necessary. In milder, protected environments, you might stretch to every 18-24 months.
The primary factors determining your schedule are the wood type and the protective product used. Dense, naturally rot-resistant woods like teak, cedar, or redwood may only need a fresh coat of sealant every two years, or even just periodic cleaning. Softer woods like pine require more vigilant protection, typically every year. The sealant type also matters; penetrating oils may need reapplication more frequently (annually) than film-forming finishes like spar urethane, which can last 2+ years but may require full stripping if they peel.
Your climate is the other major driver. Constant exposure to UV rays breaks down finishes, rain promotes mildew and swelling, and freeze-thaw cycles can crack the wood. A bench in full, blazing sun will need attention more often than one in a shaded patio.
The best practice is to perform a simple water test each spring. Sprinkle water on the bench's surface. If it beads up, the protection is still good. If it soaks in darkening the wood, it's time for a fresh treatment. Consistent, timely maintenance based on these cues is far easier than letting the wood degrade and facing a major restoration project.