It's more like every 11-25 floors, there's a boss, and each block consists of about 30-60 floors.
And in game days.
You got your facts about P3's daily schedule a bit wrong. During the day, this is the time to increase Intelligence, Courage, and Charm, and to also raise your Social Links (this also involves dating one of 5 women). During the night, if you choose, you're given the option to go to the Tartarus and go between floors to fight various Shadows and win lots of items and Yen.
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