Chapter length varies. The first 10 or so go by very quickly, but length picks up significantly afterwards. Depending on reading speed, you've got about 40-60 hours of content here to enjoy.
Afterwards, if you still want more, a replay of the game will show you just how much of the story was expertly foreshadowed. There's a youtube playlist of the soundtrack which has all music, including originals, and the discord is active with Work in Progress renders for upcoming chapters.