In reality you can add it any time during the boil as it will simply dissolve and become part of the wort. Personally I add sugar towards the last 10 minutes for really no particular reason.
That being said, IMO if it is an amount larger than a pound you would be best to make a simple syrup and add it as fermentation slows. The teas is because it is a simple sugar you are giving the yeast a chance to consume the more difficult sugars first and then the simple sugars.
In some cases with large sugar additions the yeast will consume those first and then not fully ferment the more comix sugars of the malts