It will ferment assuming you’re not using any kind of preservative. How long depends on how many and what types of yeasts, the temperature it’s being kept at, etc.
I buy UV pasteurized dessert juice (“cider” in the US at thanksgiving) in a gallon plastic jug and leave it in the fridge, presumably around 38 F. It will ferment to dry. I never timed it but it takes a few months if memory serves.
If you plan to drink it quickly, like days or a week, you’re probably fine. Keep tabs on the pressure and sugar content. If the SG starts dropping, either drink quickly or get it out.
I have 10 gallons of cider (from desert fruit pressing from fall 2020) that is completely dry. I am splitting it between two 5 gallon corny kegs, force carbonating it, and will serve it side by side from my "keezer" (once that project is completed). I was thinking with one of the kegs I would add some raineer cherry juice to sweeten it up. My question: will the raineer cherry juice ferment, or degrade while in the keg? And if so, any estimates on how long I would have to drink it before it begins to go bad (or go better?). Will the CO2 protect it from fermenting? Or protect it from going bad? Abv is estimated at 6.6