This page of the Outriders guide explains how much time you'll need to beat the game and how fast you can do that. You'll find out how long it will take you to complete the campaign with and without searching for all kinds of secrets and completing 100% of the game.
If you want to play through the main storyline only, it will take you about 15 hours just to complete it. Finding all the notes, completing all main and side missions as well as the expeditions, takes about 68 hours (up to as much as 100 hours, depending on your pace).
On average, after 15 hours of the main storyline, you will have the opportunity to start the most difficult missions - expeditions. These are the most difficult quests in the game, which have their own challenge tier.
Keep in mind that in order to complete 100% of the game, you will also have to get all the achievements, some of which require you to kill enemies using a specific method (e.g. using skills only) as well as completing all the Hunter and Wanted quests.