According to Stan's Guide it says:
"Some people have medical conditions that mean fasting has a negative effect on them. Sources say that 8 hours is needed. I would suggest individuals know their own metabolism better than a book. If you know that your stomach feels empty after 4 hours, then I suggest you have a 4 hour fast. Try to avoid eating anything large and heavy. Avoid drinking anything two hours prior to drinking the SN."
So thje recommende faasting time is 4 to 8 hours, though it seems that people usually do 8 hours based on previous threads. Two hours of dry fasting- no food or water- is also part of the plan.