5 days was just right for me. If you plan on doing the cleanse one day you should know that hunger isn't really the biggest barrier to completing the ten days. I think our preoccupation with food is the hardest thing to overcome. Don't eat for days and you'll know what I mean. A snack passes the time while you're busily working. A meeting, a casual gathering, everything we do involves food. Even your body is busy making decisions about food. Hunger pangs, eating, digesting, extracting nutrients, excreting waste, filtering fluids, it's all the time working in a constant cycle. When you don't eat, your body begins to metaphorically pout and whine. "You're not hungry" you say to your stomach. Your stomach smirks and replies, "yes, but if you love me you'll feed me". When you go days without eating, your body starts busying itself with other matters. Overall, my skin felt better, like more blood was flowing through it.
The next time I do this I will think one day at a time. It seems to work better than setting myself up for what seemed like days of deprivation. All in all, it was very enlightening both mentally and physically. I know there's a lot of bad press out there regarding the master cleanse, but I guess everyone has to decide for themselves after trying it out.