If you want to keep that warm golden-brown color instead of letting your bench weather to silvery gray, plan on sealing it about every 6 to 12 months. In my experience, once a year is the minimum for a bench that sits in full sun or rain; twice a year is safer if you live somewhere with strong UV, heavy rain, or big temperature swings.
The exact timing really depends on your climate and the sealer you choose. A light teak protector or sealer will break down faster than a heavier marine-grade finish, but even the best outdoor sealer won't last forever on a horizontal surface like a park bench seat. Watch for the signs: if water stops beading, the surface looks dry or faded, or the color starts to look uneven, it's time to clean and reseal.
Before sealing, give the bench a good cleaning with a teak cleaner or mild soap and water, let it dry completely, and lightly sand if the surface feels rough. Apply a thin, even coat with a brush or lint-free cloth, wipe off excess, and let it cure according to the product instructions. Avoid thick coats because they can become sticky or peel.
If you're okay with the natural silver-gray patina, you don't have to seal at all—teak is naturally weather-resistant. But if your goal is to keep the color, a consistent 6-12 month sealing routine is the realistic answer.