That's actually a huge point of contention in the community. Honestly, I'm not sure there is a formal, regularly updated 'devil fruit encyclopedia' book in the real world. I've seen fan wikis and fan-made PDFs, but nothing official that gets revised editions like a textbook. The canonical encyclopedia in the 'One Piece' world itself is presented as an incomplete, historical document Dr. Vegapunk and others are constantly adding to.
So, the update frequency? It's basically whenever Oda introduces a new fruit in a chapter. That's the only true 'update.' Any physical book you might find, like a databook or fan guide, is just a snapshot of the story up to its publication date. They become outdated as soon as the next arc reveals a new power. I have the 'One Piece Blue Deep' databook from years ago, and it's missing so many fruits from the post-timeskip era it's almost funny.
If you're talking about fan resources, those are updated constantly, sometimes within hours of a new chapter scanlation dropping. But for a tangible, purchasable book? You're looking at maybe one new edition every few years, if that, and it'll always be playing catch-up.