"Is it really necessary to prep for a juice cleanse?" and "How long should I prep for?"
The answers are YES and IT DEPENDS. Here's why...
If you already eat a really clean diet (no fried or processed foods, little to no meat or dairy, no alcohol, no wheat products) your body is best prepared to start a raw juice cleanse. If you are like 99% of the population that occasionally/regularly eats one or several of the items listed in italics above then you will have a more pleasant experience during your juice cleanse if you spend a few days preparing.
Case in point: I started a juice cleanse on Monday with absolutely no preparation. Friday night was lots of wine, Saturday and Sunday were barbecues with lots of grilled meats. Monday morning I woke up and didn't want to start the juice cleanse, but I pushed through. Monday for lunch I had corn chips (organic of course) and cereal w/ coconut milk. I had such an intense craving for salty carbs that I could not resist. After lunch, I continued drinking my raw juice and by the end of the day, I felt like my body was into it. Tuesday morning I felt completely different- because Monday was technically my prep day.
When you go from eating a regular diet to extremely clean foods your body is going to rebel. You can eliminate a lot of this by prepping for your juice cleanse. It is basically a transition time that allows your body to ease into the cleanse and will help eliminate a lot (if not all) of your cravings.
So how long should you prepare for a juice cleanse?
It totally depends on your diet. If it consists of regular fast food stops then I'd say 3-4 days. If you are heavy on the veggies and don't really eat processed foods that often then you can probably prep for 1-2 days and be fine.