Here's how long it will take you to get through it, depending on your playstyle.
How Long Biomutant Takes to Beat
If you want to take the critical path through Biomutant, meaning you just take the main story from start to finish, it shouldn't take you too long.
Completing the main quest of Biomutant by itself should take under 10 hours, possibly down to the six-hour mark.
If you choose to assimilate tribes into your own, it gives the player the option to skip conquest and have the other tribes surrender, saving lots of time. This being said, most won't play Biomutant like this.
If you fancy playing through the main story and doing the most important side content, you can look around the 20 or 30-hour mark.
In this, you will want to do some of the more story-heavy quests and you will also want to take on some of the toughest enemies and get to the most hard-to-reach places. Most of the collectable quests can be skipped as they aren't that essential to the base experience.
This is a really hard one to guess, as people get different things out of games but we have an approximation. If you do all of the side quests, kill all of the creatures, and full explore all the regions, you could be looking at over 100 hours of Biomutant.
The enjoyment may waver a little as you search every nook and cranny but there is plenty to do if you really want to get the most out of your experience.