Of course, everybody's different, but like a lot of things in life, this process is as much mental as it is physical. If you DECIDE that you're going to do this, regardless of temptation or whatever, you have a better chance of meeting whatever goal you set for yourself. I read somewhere recently, "Decision is the ultimate freedom."
I know I spent a week mentally preparing myself for this fast, decided I'd start with 10 days, and am now on day 15 with a goal of 21. I've discovered a lot about choice and philosophy of food (which I shared in my post "Realizations and Cravings" earlier this week).
Remember that you can drink as much of the lemonade concoction that you need, so whenever you feel hungry, have at it!
I wish you the best in whatever you decide for yourself!