In this case, I recommend you follow the manufacturer’s label text.
Traditional kvass is fermented (often in a home setting), and will contain live microorganisms, that keep unwanted ones at bay, similar to a well-established sourdough culture.
But of course there are a few caveats:
- Some manufacturers don’t use the traditional fermentation method and sell a flavored, carbohydrated drink under the same name. This won’t have the “preserving” lactobacillae and yeasts. Consider this a kvass-flavored lemonade.
- Traditionally fermented kvass that still contains active starter culture will continue to ferment - which is not a good idea if you want to ship a product. Like for beer, the manufacturers will ensure that the activity is stopped, which again means that the preserving effect of the culture is lost. (You will get some of the yeast though, unless it’s filtered.)
- Even if someone shipped the still-active brew, there is no way to ensure that they are still alive by the time the consumer opens the bottle.
In short, while your home-fermented drink will keep for a while, the safe approach is to stick to the instructions.