Basically you can pick any 168 consecutive hours in the windows that goes from March 6 to March 14.
Let's say you start at 0.00 on March 6, your deadline is 23.59 March 13. If you start after 0.00 March 8 you'll have less than 7 days [or less than 168 hours] because March 14 deadline is the end of the challenge anyway.