Hi!
Jenavieve, you've chosen a very interesting topic for your Science fair! According to my research on your project, cutting the plant isn't necessarily involved. At first, you should let it grow the normal way until it's two centimeters long and record your observations over time. Then, you flip it and continue to observe the root growth. If you notice any significant changes that occur as a result of the direction of gravity change that is when you can stop your experiment. It's up to you! When you feel that you have gathered enough data for your experiment, that is when you can stop.
Resources:
http://www.bioedonline.org/lessons-and- ... ot-growth/
https://today.uconn.edu/2009/08/the-eff ... velopment/#
Let me know how it turns out! If you're still confused feel free ask more questions.
Hope that was helpful!
Peace