6 hours = 1200 shirts = 200 shirts per hour
9 hours = 1800 shirts = 200 shirts per hour
1 day = 1800 shirts
2 days = 3600 shirts
3 days = 5400 shirts
4 days = 7200 shirts
5 days = 9000 shirts
6 days = 10,800 shirts
7 days = 12,600 shirts
Your answer is 7 days