Octopath Traveller is a monster of a JRPG.
Not only is the game filled to the brim with content, the story offers eight unique campaigns based around the eight in-game characters for players to take part in. All eight with their own end-game boss fights. It's a crazy amount of eights.
RPGs usually need you to get ready for the long haul, but this game is still a behemoth.
How long in the main quest?
If you were to skip every cutscene, grind for as long as needed, and do every character's main story you could beat the game within 40-60 hours depending on personal skill level.
This is an estimate based on players who just rush the game to completion and power through. It is possible to rush through to a boss without fighting every enemy in sight, praying that the RNG gods are on your side, but playing this flippantly will miss out on a lot of the magic.
How long to beat the main quest and the side quests?
If you plan to take your time and enjoy everything that the game has to offer, tackling all of the main side quests throughout the world, you are probably looking at around 90-110 hours in total.
There is a lot to do and the world is truly huge, so if you plan to see it all and find everything the game has to offer, prepare to be in it for the long haul.
How long to beat everything, including the post-game stuff?
Spoiler warning: Huge spoilers for the post-game below. If you don't want to ruin your experience, take heed.
Beating the eight end-game bosses and completing every side quest will not fully complete the game. There exists a considerable post-game dungeon that players can explore called the Ruins of Hornburg.
As you'll need to grind up your levels to near-maximum and prepare yourself by getting the best weapons and armor, you are looking at around 120-150 hours.