Fonty
1. The average weight lost over a long term fast is about a pound a day. It could be more or less depending on your starting weight, more if you are obese, less if you are skinny to begin with. The weight lost isn't as important as what you are losing, excess fat, toxins, damaged flesh etc. And you also definitely need to take into account the refeeding time and allow for some weight to be put back on (minimal it's true) to restock your at least partially depleted body of nutrient stores.
2. The only time limit to a fast is the amount of stores that are in your body. It is always okay to 'fast to completion' if you have no contraindications to fasting. Many fasting experts suggest that is the best if not the only way to conduct a proper fast.
3. For a medium long term fast such as you are contemplating I would suggest it would be far better to complete the full recovery/refeeding period before beginning a new fast. So after a 21 day fast, refeed on mostly raw food for 21 days afterwards, restocking your body with good nutrition and keeping the body clean with the high water content raw foods to prepare yourself for your next fast.
And read this:
http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.toc.htm
And anything else you can find about fasting. Then if you have any further questions let us know.
Good luck!
Andrew.