I had bearing buddies which I used all the time on a boat trailer some years ago . However, at the end of each season, I pulled them apart and always found at least a little water in there.
To answer the question of how often I do my current trailers ( none of them boat trailers ) I shoot for every three years. But then I am retired, so at least according to my kids and grandkids, I have all the time in the world to do non-essential things. Yeah right.
With these cheap-azz drum brakes they fit, I'm pulling them apart more for brake inspection than bearing maint. Upfitting to disk brakes would probably be a good idea for my trailers.