Arthur had only 36 hours to spend in the Minimoy world, and he leaves at midnight. He spends a full 24 hours too long as a Minimoy. After he finds the Minimoy Princess they travel during the day. They spend the first night sleeping in a flower (24 hour mark); the second night he is wandering under the house, the grandma is sleeping in a chair above him (48 hour mark). He then returns at noon. That's 60 hours.