"Is it really necessary to prep?" and "How long should I prep for a juice cleanse?"
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 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 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 prep for? 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.