* Your experience with the game: If you're a seasoned Pokemon Mystery Dungeon player, you'll likely be familiar with the mechanics and strategies, which can speed up your progress.
* Your chosen difficulty: Playing on a harder difficulty will naturally increase the amount of time you spend on the game.
* How much you explore and side quest: The game offers a lot of side content like optional dungeons, quests, and even a "Mystery Dungeon" mode that allows you to create your own dungeons. How much of this you engage with impacts playtime.
* Your efficiency in battles and exploration: If you're skilled at navigating dungeons and defeating enemies, you'll naturally finish the main story quicker.
Based on various player experiences, a reasonable estimate for completing the main story of Pokemon Mystery Dungeon Explorers of the Sky is around 20-30 hours.
However, if you want to fully experience all the game has to offer, including side content and post-game challenges, you could easily spend 40+ hours or even more.
Ultimately, it's up to you and your approach to the game to determine how long your journey in Miracle Sea will take!