In the May 31, 2007 article "Notes on a Food-Free Diet" by Joel Stein in Time Magazine, the author does the Master Cleanse for only 2 days even though the instructions say do it for a minimum of 10 days.
There's a reason for the 10 days. Typically days 2, 3 & 7 are somewhat unpleasant due to detoxification symptoms, such as those described by Joel Stein: lightheaded, irritable, bored, trouble sleeping. As they say, "No pain, no gain." However, the other days are usually very pleasant and filled with energy.
Doing the Master Cleanse for 2-days is like having your doctor prescribe 10 days of antibiotics for an infection, taking them for 2 days and then saying they don't work.
It is interesting to note, however, that even though he does it for only 2 days, Stein talks about seeing how much of our day-to-day life revolves around food, feeling superior to food, and not being hungry.