Sealing your external timber is a lot like washing your car. There is no one correct timeframe.
There are many factors that speed up the need to seal your timber like - direct sunlight, harsh weather conditions like high wind areas, snow, rain or extreme heat.
Another key point to consider is that is it a high traffic area? Generally speaking every 6-12 months on high traffic areas and areas that see a lot of sunlight. You will be able to notice when the boards start to dry out or discolour.
The more you can clean and do a single coat the healthier your timber will be. This will then avoid you having to sand the whole thing back and a 1 day job turns into a few days or a week job.
Our go to product is @cutek_global CD50 having great success in long term external timber care. The reality is no one likes sealing or maintaining their external timber so if you can look after more frequently, you will avoid spending more time and money in the long run.