For a completely different angle, look into dramatic audio productions from companies like Focus on the Family Radio Theatre. They did an acclaimed, fully-dramatized series for Narnia with professional actors and cinematic sound design. It's more like an old-time radio play than an audiobook. These are often sold on Christian bookstore sites or general retailers like Christianbook.com. Even if you're not religious, the production quality is top-notch and it's a incredibly engaging way to experience the story. It feels like you're right there in the snowy woods with Lucy.
You know, I've never actually timed it. I just know it takes about as long as driving to my grandma's house used to take. It was our traditional 'car tape.'
We'd start it as we left the city, and Aslan would roar just as we turned onto her street. It was like clockwork. So for me, it's not 4:22, it's 'the length of a trip to Grandma's.'
I suppose that's not a helpful answer for everyone, but it's the truth! The runtime is woven into my personal history, not just my media library.
For a nuanced take, I'd say the lower bound is about 6 for a receptive child listening with a parent, and there's no upper bound. The themes are eternal. The scene where Aslan explains 'deeper magic from before the dawn of time' gives me chills as an adult in a way it never did as a kid. An audiobook narrator who nails that moment can make it profound for any listener over the age of, say, 12.
It's also a great 'gateway' audiobook into fantasy. It's not overly complex with dozens of factions and made-up languages. It's a single, compelling journey with a clear endpoint. That makes it manageable for a younger listener. After this, they might be ready for 'Harry Potter' or 'Percy Jackson' audiobooks. So age 8-10 is a perfect window to use it as that bridge.
LibriVox has public domain audiobooks read by volunteers. Unfortunately, C.S. Lewis isn't public domain yet in most places, so you won't find Narnia there legally. That site is amazing for older classics like Jane Austen or Sherlock Holmes, but for 20th-century works, you'll need the paid or library routes. Just a heads-up to save you time searching there.