I'm not familiar with the brand or how to test if the wood needs recoating, but below is a link to a previous discussion of how to use the product.

Here is a link that might be useful: SRD

Wood looks good but be careful with Sikkens. It is made from Linseed oil which has a tendency to allow mold and mildew to form because of the sugars. So I try to avoid it. It's also a bear to strip off in preparation for a new stain. These are the considerations that many homeowners don't think about when choosing their stain. I prefer petroleum-based products so I get a deep-diving penetrant rather than a surface coating and they are easy to clean and prep for maintenance.

If you stay with the Sikkens....clean for your fall maintenance and reapply to keep your board protected. I am a firm believer in maintaining your wood!