Spoilers about the main story's time length and the final boss' level below:
A twitch streamer was able to finish the game's main story in 43 hours. He may have rushed through it though. Final boss was at level 50. I got this info from the chat room though because i personally didnt see him finish the game. I did see him triggger more cutscenes and mini boss battles in the post game though. I'm guessing the boss battle was from a side quest he didn't finish or is post game content. The chat also claims that the streamer has seen strong enemies at level 99. They're probably like the super enemies like xenoblade.
The twitch steamer is hamayama if you want to check it out yourself. Beware of post game spoilers though.
edit: Nvm looks like he's done with streaming the game since he completed the main story.
