It really depends on the wood, the finish, and how exposed the piece is. For most wooden urban outdoor furniture—balcony benches, courtyard tables, rooftop chairs—I’d inspect the finish every 3–6 months and plan to reseal every 12–24 months. If the piece is fully exposed to sun, rain, wind, and city pollution, every 6–12 months may be more realistic. If it sits under a cover or awning, you can often stretch it to every 2–3 years.
Don’t reseal on a calendar alone. Do a simple water test: if droplets bead up on the surface, the finish is probably still doing its job; if water soaks in quickly or the wood looks dry and faded, it’s time. Also watch for rough grain, grey patches, small cracks, water stains, or mildew. Those are signs that the wood sealer has worn thin.
Before resealing, clean the furniture with a mild wood cleaner and let it dry completely. Lightly sand if the surface feels rough, then apply the same type of product—oil or film-forming finish—unless you’ve checked that a switch is safe. Don’t over-apply, because too many layers can trap dirt and moisture.
In urban settings, grime and pollution can make outdoor wood care more demanding, so keep the furniture clean and inspect it regularly. If you’re unsure, annual inspection is the safest habit; your reseal frequency will settle into a rhythm based on your climate and exposure.