Resting is good during a water fast, its helpful to the fasting process.
As for how long the fast will be - you won't go beyond the return of hunger which can vary in length for each faster and each fast.
It depends on how much nutritional reserves a persons body has and the degree of work to be done by the body during the fast.
Here's some reading on the subject and a quote from it. http://soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch23.htm
"" A 'finish fast' does not always mean a long fast. It does not mean a fast until all of the body's food reserves are exhausted. It is a curious fact that hunger will return in three days, even where there are abundant reserves on hand, if three days are all that are required for the patient to get well; whereas, it will not return for five weeks or longer, even where there are fewer reserves on hand, providing this time is required for the body to eliminate its accumulated toxins. Fortunately, in most cases it will be perfectly safe for the patient to fast until hunger returns.""