Fasting til completion is when you continue until the return of natural hunger is present.
The experience is usually very distinct in the throat area, as I have read.
I have not experienced it myself but that is apparently the key indicator of when the fast is complete.
Fasting till the return of natural hunger.
This is the point where the body has finished either all the work, or everything that is going to be accomplished for that fast.
It occurs when the nutritional reserves of the body are at a point where it is time to discontinue, or the body has completed everything it needs to work on.
I don't know that you can get into trouble if you follow this guide.
Beyond this point is where the body starts to use up vital tissue for its sustenance.
This is where the return of hunger interrupts that possibility.
Before that point the body is using up and living off of every bit of junk that is not needed and also using its stores of nutritional reserves.
Personally I think that if you do increasingly longer fasts that you will be able to sense the safety of your fast.