Question
how often should you apply sugar  for dallis grass ciontrol, seems to be working

Answer
It is important to remember that any product (natural or synthetic) that will kill a weed (a species which is not wanted) is a plant killer. That means that it has the potential to kill other species.
"Paracelsus, sometimes called the father of toxicology, wrote:[21]
German: Alle Ding' sind Gift, und nichts ohn' Gift; allein die Dosis macht, da?ein Ding kein Gift ist.
All things are poison, and nothing is without poison; only the dose permits something not to be poisonous.
Or, more commonly
The dose makes the poison."  (Source: Wikipedia).
I have no doubt that sugar will kill some weeds.
As far as I can tell, no one has said which weeds.
If I wanted to find out, I would have to lay out a statistical field experiment using different rates and a control.
That has all been done for me by the manufacturers of synthetic weed killers.