Other than “once it’s been written and polished,” when is the best time to launch a new book?
Self-Publishing Review offers some general release-date guidelines and also shares one trad-pub expert’s tie-in calendar (which focuses on when buying picks up, not when releases are scheduled) to give you some food for thought.
All that said, of course, if your readers are clamoring for your next book, ignore those suggestions and just get it in their hands once it's ready!