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Not sure the exact time frame, but I think it only took me a couple of hours, maybe 3 at most. Your milage may vary, but even though I was annoyed at the level 4 community requirement when I saw it, I managed to hammer it out pretty quickly, much quicker than I was expecting. In addition, after completing some quests, you unlock multiple community members instead of just one.
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I'm here to report that it didn't take nearly as long as I thought to complete, since the side quests are all pretty short for the most part. Knowing what I'd read about people's reactions to the Torna ending, I decided I was going to press on, even if it took a while, and even if I only played it in little chunks until completion. I felt exactly the same way when just after finishing the quest to get to community level 2, the next main quest was to get to level 4.
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