Depends heavily on what you decide to do in game, as well as how fast you read. Someone who only follows the story progression and reads extremely quickly can probably get to the end of current story in under 2 hours. However, if you decide to spend time doing all of the optional things and taking your time reading through the story, you can drag it out basically as long as you want. There are repeatable scenes as well as the fishing minigame which can eat a lot of your time.
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I tried speed running it from the beginning takes around 4-6 hours if you have the skip unseen text option on then its around 2-4 hours I haven't tried it tho that's what I assume if you play it normally then it can last for a few days I suggest savoring the game has a very fun story I suggest reading through it instead of skipping