That's an excellent question, and the honest answer is: it depends more on the look you want than on a strict calendar. Teak is a remarkably durable hardwood, and from a purely functional standpoint, it *never* needs oil or sealant to survive outdoors. Its natural oils protect it from rot and insects. However, most of us want to maintain that beautiful golden-brown hue, and that's where maintenance comes in.
If your goal is to preserve the original rich color, you may need to apply a high-quality teak oil or sealant as often as every 3-4 months during the peak sun and rain season. This is a high-maintenance approach. For most people, a more practical and common frequency is once a year, typically in the spring. This annual treatment helps slow the natural weathering process to a silvery-gray patina.
The best indicator is the wood itself. If the bench looks dry, has lost its luster, or is turning a patchy gray, it's ready for care. Before any application, ensure the wood is completely clean and dry. Use a proper teak cleaner to remove dirt and mildew. Apply oil or sealant evenly with a brush or cloth, following the product instructions. Remember, oiling darkens the wood, while sealants (often labeled as "protectors") may allow more graying but offer longer UV protection.
Ultimately, the choice is yours. Embrace the low-maintenance, elegant silver patina, or enjoy the warmer look with annual maintenance. Either way, your teak bench will last for decades.