Half of it is main story and the other half is additional content.
God of War was roughly 40 hours long for most people who finished the main story and did most side quests and other things. And it seems like God of War: Ragnarök will offer something similar. Insider Gaming reports that the main story of the game is about 20 hours long, with 16½ hours being gameplay and the rest being cut scenes.
If you are a completionist, there will be an additional 20 hours to enjoy, with 19 of those being gameplay (and 1 hour cut scenes). All things combined, it looks like we have 40 hours to spend with Kratos when God of War: Ragnarök launches for PlayStation 4 and PlayStation 5 on November 9.