I’m on Day 7 of intense dialogue immersion as I draft that chapter of Writing Young Adult Fiction for Dummies, so your question finds me in the right frame of mind—albeit barely. The gray matter is nearly wrung dry on this topic. Let’s see what I can squeeze out.
There’s no official “too much” threshold for dialogue in YA fiction. You’ve got to find the right balance of dialogue and narrative for your style and your target age group. The bestselling The Book Thief (ages 12 and up) is almost 600 pages, with probably 2/3 of each page being narrative rather than dialogue. This might intimidate younger readers, who tend to feel comfortable seeing white space and dialogue on their pages. But then, The Book Thief’s got a lot of white space thanks to frequent paragraphing, and its conversational narration makes even the narrative bits feel like dialogue, establishing a satisfying balance. The reverse, a book that’s 2/3 dialogue on each page, can feel balanced if the narrative that does appear avoids wasting time on innocuous actions (brushing hair aside, turning to face other characters) and instead offers dynamic and revealing actions that challenge readers—perhaps deliberately contradicting the spoken words, or hinting at feelings that the speaking character wants to hide. The narrative could add a subtext, extra plot info, and additional tension to the story. My worry is that setting might be overlooked when the narrative is spare. Setting can be worked into the action (a character interacting with a prop in a revealing manner) or directly addressed in the narrative (a brief sensual description of the place) to illuminate or enhance your character in ways that dialogue alone cannot do.