The short answer
There's no single shelf life for alcohol-free wine that applies to every bottle. Unopened, go by the best-before date and the storage instructions. Once opened, what applies is whatever the manufacturer states for that specific product regarding chilling and use-by time.
Unopened: best-before date and storage go together
The best-before date, or BBD, is mandatory. The EU Commission explains this requirement for dealcoholized wine products under 10% vol, meaning less than ten percent alcohol by volume. The responsible producer must set the date, since they know their product. Neither a marketing claim nor the bottling date can substitute for this information.
The best-before date applies to the unopened bottle, and only with proper storage. Both conditions apply together. A spot in the cellar therefore isn't automatically suitable: check what conditions are required. A hot car and a sunny windowsill are, over time, the worst places for a bottle.
Why manufacturers state such different time frames
Carl Jung's data sheet for its Merlot states a shelf life of three years from production. This figure applies to that specific product. It says nothing about an opened bottle. SOBERCIETY recommends, for its Riesling, cool, dark storage after opening and consumption within a few days.
Behind this lie two quite different questions. First: how long does the sealed bottle keep under the intended conditions? Second: how should you treat it after you've opened it? You can't derive a reliable time frame for the opened contents from a yearly figure meant for the sealed bottle. Once air reaches the contents, a different calculation applies.
A practical routine after opening
Read the instructions before you pour for the first time. Write the opening date on the bottle, for example on a sticky note. Then reseal it and store it as cold as the manufacturer specifies. Pour out only as much as you need right now. Leftovers from a used glass don't belong back in the bottle.
If there's no time frame on the label, ask the manufacturer about this specific product. Generic internet tips like "always keeps for a week" are no reliable substitute. They know neither the recipe nor your fridge. If you only drink a glass occasionally, a smaller bottle size is often the more convenient choice.
What matters additionally for sparkling varieties
Sparkling bottles have an extra issue: the carbonation fades. A flat glass isn't much fun, even if everything else is fine. So use a stopper designed for sparkling wine. It holds the pressure better than a simple cork. The chilling instructions still apply. Such a stopper also doesn't extend any time frame stated by the manufacturer.
For a reception, it's better to open bottles gradually. Opening all of them at once will backfire by the end of the evening. Keep a fresh bottle ready and only open it when it's needed. This helps avoid leftovers and stale pours.
Can you tell if wine has gone bad?
If something smells or looks noticeably different than usual, don't keep pouring it. Noticeably altered packaging is also a reason to leave the bottle alone. The reverse doesn't hold, though: an unremarkable smell proves nothing. An improperly stored bottle can smell normal and still no longer be suitable. If the storage history is unclear, don't rely on a test sip as a safety check.
For future planning, a quick note helps: how much was actually drunk? If a large amount is regularly left over, change the bottle size or the quantity you buy. The best shelf-life strategy is usually to open only as much as you actually need.