There are no rules per se, however I added on 8 days of juice fasting after 17 days of MC.
I wanted to break on a weekend, so that's how I came to 8 days.
I think it's best to go by gut feeling. On day 17 my body was screeming in rebellion against continuing the MC. But everybody is different.
The idea is to do 3 - 6 cups of juices plus lots of water. I ended up with about 7 cups of juice a day. You continue the SWF (Salt-Water-Flush) to have regular BM.