Unless you can afford to check into a fasting clinic, it's up to you and you alone.
Setting reasonable goals helps. You may prefer Water Fasting as there may be less appetite than when you are tempting your stomach with juice. Also, Water Fasting works at least twice as fast as juice, so you won't be challenging yourself as long.
A 12 day Water Fast followed by a few days of juice and you may just reach your goal.