What's the right length for your book? The answer is simple. There is no "right" length. You must research the marketplace. There are clear trends within it that will help you decide what works for your book.
To get a basic sense of what works in terms of sales, it's important to check out best sellers. Three popular list sites include: The New York Times Best Sellers lists, Indie Bestseller Lists, and USA Today's Best-selling Book List. Perusing these lists makes it clear that many titles have three or fewer words. Shorter titles are the norm.
Don't stop with studying the length of best-selling books. Look at titles similar to what you want to write. You will find extensive book databases at Goodreads.com and WorldCat.com. Both of these sites list most books published in every genre. Take time to list 10 books that are similar to yours. What is the length of each title? Your book title length should be somewhere in the neighborhood.
Fiction titles tend to be much shorter than non-fiction titles, but only because non-fiction titles often include subtitles. As a rule of thumb, a non-fiction title should be 5 words or less. Subtitles are generally three to seven words in length. For non-fiction, the title is shorter and the subtitle is longer. In non-fiction if you choose a long title, your subtitle must be longer. Try to choose titles with subtitles that have up to a balance of 10 words. Remember, subtitles are rare in fiction. Here, titles are often 1 or 2 words, 3 words is also common, but overall, fiction titles are 5 words or under.
There is one more special case for title length. Some authors write multiple books with similar titles. Each of their titles has the exact same word count or a very similar word count. This works well for authors who write series and may fit your work.
For more title length help, Title Generator by Aabashenya may assist you in creating a strong title that is a perfect length.
In closing, the most important thing to takeaway about title length -- shorter is easier to recall.