Interesting that this comes up today. I'm refinishing a redwood slab that is an irregular live edge piece 3" thick and overall about 8' long. This is the 2nd go round on this because the customer didn't do didley covering it up or cleaning it. Lasted 3 years (well, it didn't last 3 years or I wouldn't have had to strip it down to bare wood) but needed refinishing. I'm thinking it was probably OK to just re-coat it after 2 years, but once a year would be sure thing. And that's not covered.
I use
Epifanes Clear Varnish and follow the instructions to a T.