I've tried a juice fast on 2 occasions only to find myself hangry and uber lightheaded during my workouts. So guess what? I'm going to eat food.
I think a freshly pressed juice is great for a snack, but for me, I'm too active (burning 700-1000 calories per workout) to just drink juice all day, every day for 7 days or more.
Have you juiced? Do you want to juice? Why are you planning to juice? Those are the questions!