You’ll need about 85 to 90 pounds (39 to 41 kg) of fresh grapes (still on their stems) to make five gallons of wine. It will start off at about 2.5 lug boxes of grapes and will end up as about 25 bottles, or two cases, of wine. The final volume of wine you get depends entirely upon how much juice you squeeze out of the grapes, whether you press before fermentation as in a white wine or press after fermentation as in a red wine. The final volume, therefore, will be affected by all things that affect the quantity of juice recovered from those lug boxes. For example if there are many tiny berries per grape cluster, you can assume that there will be a higher percentage of solids/liquid, both in the form of skins and seeds and in the grape stems that house each of those tiny berries. In large, loose clusters where the stem/berry ratio is less and the skin/juice ratio is less, pound for pound you will recover more liquid from those grapes.
We could be splitting hairs, here, however. Even though there are varietal to varietal variations on berry size, stem weight, and other factors, it’s almost impossible to accurately gauge how much juice will come from a certain lot of grapes unless you have years of experience working with grapes and know what volumes they have yielded in the past.
Another monkey wrench to throw in is the year-to-year variation in processing parameters as well as the winemaker-to-winemaker differences inherent in the winemaking process. Your friend down the road may take 100 pounds of grapes and press the heck out of them to yield seven gallons of wine, whereas your winemaking regime, with its gentler pressing, may yield only five gallons.
Unfortunately there is no easy answer to your question. We are at the whim of Mother Nature, who one year may make your Cabernet vineyard full of fine, heavy clusters when one year she may make half the berries per cluster too tiny to yield any juice at all. All a winemaker can do is make estimations, gain experience, and go from there.