Honestly, I got about eight hours in and switched to the book. The audiobook's pace was too slow for me, and at 21 hours total, I calculated how many other books I could listen to in that time. I know that's sacrilege to some fans, but the density of the prose made it hard for me to absorb in audio. I needed to be able to re-read paragraphs. So, length-wise, it's long, but your mileage on enjoyment may vary.
It's Rob Inglis for the unabridged novel. I see some confusion sometimes with the fantastic BBC radio play, which has a full cast including Ian Holm and Michael Hordern. That's an adaptation, though. For the pure book experience, Inglis is the definitive narrator. His voice is synonymous with Tolkien's world for an entire generation of listeners.
I've listened to both, and they're like two different genres almost. Rob Inglis makes it feel like a classic, maybe even a textbook in the best way. Andy Serkis makes it feel like an edge-of-your-seat adventure serial. I prefer Serkis for the simple reason that he keeps me awake during the Council of Elrond.
His Gollum is, unsurprisingly, mind-blowing, but so is his Samwise. You can hear the loyalty and the heartbreak. It's a stunning performance that adds new layers even if you know the story by heart.
The practical answer: Go to Audible.com or open the Audible app. Search 'Return of the King'. Buy it with a credit or cash. Download. Listen. It's a solved problem. Spotify's entry into audiobooks is too new and fragmented to rely on for a specific, major title. Maybe in a few years it'll be different, but right now, Audible is the de facto store for this. Don't make it harder than it needs to be. The story is the prize; the platform is just the door.