It took me around 80 hours to clear the whole story for Metaphor: ReFantazio. This time will likely vary for you, depending on if you're struggling with some of the tricky bosses and whether or not you opt to lower the difficulty.
Due to the game's calendar structure, you will always have time to spare between completing a story dungeon to engage with side content that can improve your Royal Virtues (like Eloquence), rank up your relationship with Followers, or complete optional dungeons and side quests to gain more experience and rewards.
There's no post-game once you finally complete Metaphor but you will be able to save your cleared data, which will allow you to start New Game Plus, which carries over certain progress from your previous playthrough.
While you should have enough time to complete all side content on your first playthrough, New Game Plus is recommended if you wish to unlock and master every single Archetype.
Collection
[
|
...
]