Depending on the type of red wine you are making, fermentation should continue to the appropriate level of dryness. If you are making a dry red wine like Cabernet, fermentation should carry on until most of the sugars have converted. Use a hydrometer to measure the Brix, which tells you how much sugar is still in the must. The lower the Brix, the more alcohol and less sugar there is.
To do this, take a sample of your wine and put it in a container like a jar, be sure to include all portions of the must, stem, seeds, skins if any. Let the must settle for a few minutes until you see the meniscus form (the thin layer on top of the must, it looks almost clear). Use a thermometer to get a temperature reading. Then put the hydrometer on the bottom part of the meniscus and spin it a little to remove any bubbles. Record the number on the hydrometer. That will give you the level of sugars remaining. Then it’s all a judgement call from there; basically what tastes good to you.
The simple rule of thumb is to divide the Brix reading by two. For example, grapes at 24 degree Brix equates to wine with 12% alcohol.
Cheers,
Phil