1.5 times duration of previous fast is a good rule of thumb.
I had a lot of personal email exchanges with chrisb1 and only thing I have from Chris is what he said about my kidney cyst......"Archus, kidney cysts are usually not harmful, but could be annoying, if you really want to get rid of the cyst, you may fast for 30 days, re-feed for 30 days and then fast again for 30 days. Sometimes it takes two back to back fasts to clear up large cyst and other health conditions."
His last email came to me on October 09, 2012. I have emailed Fonty to find out if he knows anything about Chrisb1.