You need to do it for at least 5 days, according to my acupuncturist, but the full 10 days is definitely preferable. He recommended that I start with 5 days as the goal b/c it seemed less daunting, and that by day 3 I'd be fine. I was going through a lot when I tried it, so I only made the 5 days, but I think I could do the whole 10 now. He also said it can't hurt to do 5 days (or longer, as much as you can) the first time and then if you can't do the whole 10 days, try again in 6 weeks.
What really helps is doing the salt water "cleanse" the day before so your system gets cleaned out as much as possible. It's gross, but if you do it at the beginning, you're only tied to the bathroom the first day and then after that, you'll still need to pee a lot but at least it won't be gross.
Also, if you're feeling hungry, you're not drinking enough. That was my problem with it -- don't go by whatever amounts the program says. If you're hungry, drink more lemonade!