According to the book don't exceed 40 days. Although people do go beyond that time with varied results. I have read posts on another board of people doing 100 day with juice...which is a lot different than the Master-Cleanse nutrient wise. If yo went that long on the MC, I would hate to think what happens to the liver and kidneys processing all that maple syrup over such a long period of time. I would think in time you would get diminishing results and you would be better off splitting your cleanse into maybe two "shorter" ones to allow a rebuilding cycle. Maybe do 20 days if your body allows it and then do another 20 days a couple months later after eating healthy.
One final thing that is important. The 40 or 10 day time limit is only a guideline. Any faster should allow the body to dictate when to return to food...and that is when it feels genuinely hungry. That might be on day 9, 12 or even day 42. If you follow a certain time structure then you might not doing your body good.