Rymar Extreme has been primarily a commercial product for years. You can now get it at Sherwin Williams. It is not cheap, $70-80 per gallon.
I have used it on my siding and deck for the last 4 years. I just applied my second application.
On the vertical surfaces of my deck, after 4 years, I had no need to reapply. The stain still held its color and coverage very good. On the floor however, I started to notice wear/fade at year 2. By year 3 the deck needed a new coat. I waited till year 4 thinking by that time the vert would need it.
Just last week I pressure washed and cleaned the deck, applied 3 days after a dry spell. Looks as good as new. I am a big fan of this product.
Based on the vertical wear my deck showed, I expect to get all of 5 years on my siding as it has limited exposure to any intense sunlight. I am on year two and it looks good as new.