The short answer is once or twice a year for most wooden park benches. Spring is the best time for a full coat, because the wood tends to be dry enough to absorb the oil well after winter. If your bench sits in full sun, heavy rain, or coastal air, add a lighter second coat in early autumn. In really harsh climates, you might need to reapply every three to four months, but that is the exception rather than the rule.
Try not to oil on a strict calendar alone. Watch the wood itself. If water no longer beads on the surface, or the grain looks dry, faded, or slightly rough, the bench is ready for another coat. A bench under a covered area may only need oiling once a year, while a bench that bakes in afternoon sun may need two or three coats.
Before applying protective oil to a wooden park bench, clean the surface first. Use warm water, a mild wood cleaner, and a soft brush to remove dirt, bird droppings, and mildew. Let the bench dry completely for a day or two. Then apply the oil in thin coats with a brush, pad, or lint-free cloth. After about 20–30 minutes, wipe away any excess oil. Thick coats do not protect better; they just leave a sticky, dust-attracting film. Two thin coats in the first year, followed by a maintenance coat once or twice a year, works for most benches.
The type of oil matters too. A penetrating oil with UV inhibitors helps in sunny locations, while a mould-resistant formula is useful in damp or shaded gardens. Hardwoods like teak or iroko usually need less oil than softwoods like pine or spruce. Some people even leave teak untreated so it weathers to a silvery grey, but if you prefer a rich brown look, oil it lightly once a year.
So start with once or twice a year, check the surface every season, and let the bench tell you when it is thirsty. If it looks dry or water soaks in quickly, it is time for another coat.