I did some research but didn’t come up with the answer. I made another gallon of syrup the day before yesterday, but have decided to possibly quit feeding the girls as the alfalfa will be blooming in the next week or two. I would like for them to finish out the frames in the brood and get the second brood box on, they still have a frame and a half left, but think they should start fending for themselves. How long can I keep what I’ve already made before it goes bad?
Ideally you should be adjusting the pH to 3-4 at which point between the sugar content and the acidity it should last almost indefinitely.
What would be good to put in the syrup to up the pH?
Citric acid powder or lemon juice
I use food grade phosphoric acid that I had around for adjusting water pH
A touch of “Apple Cider Vinegar”
Refrigerated it should last indefinitely.
As long as you like if you put it in the freezer. 
Hello I have just made up 8 litres of sugar syrup to prepare for Autum.
Does anyone know if there is a particular herb or natural remedy to prevent sugar syrup from fermenting ??
Thymol will do it. Bit of a fiddle to make up, but it works. The other choice is 1 teaspoon of household chlorine bleach per 4 liters of syrup.
If you choose Thymol, here is @Dee’s recipe - add 1ml per liter of syrup to prevent fungus in the syrup: