How about 2-3 years between coats? Depending on where you live of course. I recommend a yearly maintenance coat to be safe.
I live in the Caribbean and we usually have to re-coat every 6 months to keep regular varnish up good. And that is with a start up of at least 8 coats if not 12.
2 things that are against this “varnish”
- Tendency to NOT do a yearly maintenance coat because it looks so good. If you let this stuff go it’s hell to pay striping it off.
You can read more on my blog and see some photos too.