You can safely continue on until there is a distinct sensation of real hunger.
It will be experienced in the throat area and unmistakable. - You won't miss it.
During the fast there shouldn't be any desire for food after the second or third day.
When real hunger returns, your body has used up its nutritional reserves, or has completed all of the healing cleaning and repairing that it is going to do for that fast.
Natural hunger can return anywhere from a few days to fifty or more days.
Its different for everyone and different for each fast.
Don't worry, if you become so weak that its hard to do much you will probably discontinue the fast, even though weakness is not the indicator to stop.
So just go for as long as you need or want and you will stop when you have had enough or the hunger speaks to you.