The text of The Master Cleanse recommends a minimum of 10 days, so I'd go with that, especially if it's your first cleanse.
I know every day that I continue on (I'm on day 14), I feel lighter, cleaner and better. I think relying on the text for guidance, and listening to your body and how it feels, you'll know when it's time for you to stop (the max recommended by the text is 40 days). I'm not sure about a change in bm color, etc.
As for being concerned about losing too much weight, the text says - and is further substantiated by posts from experience here - that you can adjust the amount of maple syrup to more or less depending on how much energy you need and/or how much weight you want to lose or not lose. So your boyfriend can use more if he feels the need. The only element you're not supposed to adjust is the amount of lemon juice.
Congrats on your success so far, and much luck with the rest!