A good measure is whether or not you feel that empty, churning feeling in your stomach. Another good measure is if you're into the idea of eating something like plain cabbage or oats.
I feel that after a few hours of not eating. Is feeling it or not feeling it the sign that I've reached the necessary effect?
I have no personal experience with this, so I must have read I somewhere. Usually in the first hours your get hungry, which eases off after some time. After that, you can until you are hungry again.
I think it is bare minimum for good clean digestion. As far as ALL the benefits of fasting... I think it is a good start.