The 6-1/2-month rotation is completed by doing each week twice before you move on to the next week. So each Mesocycle takes 8 weeks to complete, followed by a recovery week.
My first time through STS I did the extended rotation. Since it was all so new to me I felt it gave me a really good way to run through the workouts the first time and then the second week, since I was more familiar with everything, I got a little bit better workout because I knew what to expect and could concentrate more.