Minimum number that is "tens of thousands of hours": 20,000
7 years * 365 days per year = 2,555 days
20,000 hours / 2,555 days = 7.8 hours per day
The typical Western working adult's weekday schedule:
8 hours work (work 9am to 5pm, which is 8 hours, minus 1 hour for a lunch break, plus 1...