It depends on the sugar level. Each yeast strain has limits regarding how much alcohol it can tolerate -- if there is sufficient sugar, yeast can produce enough alcohol that it poisons its own environment. For commercial wine yeasts, the tolerance ranges from 10% to 18% ABV.I’m I right in thinking the longer I leave it the more the sugar will turn to alcohol, or is that not true?
The recommendation for home winemakers is to plan to ferment wines dry. Start the wine where the SG will produce the desired ABV -- tables are available that provide a correlation between SG and the approximate ABV.
Use a yeast whose tolerance level is above the target ABV. If you have no idea what to use, choose EC-1118, which will ferment a rock if given the opportunity. It normally can hit 18% ABV, and members have reported step feeding it to a higher ABV.
Let the wine ferment dry, and at bottling time, stabilize with potassium sorbate and K-meta, and backsweeten to taste.