There is no hard and fast rule, if you follow most recipes you will be safe, it really boils down to the sugar/yeast ratio or sugar/alcohol ratio. You can get kits to measure the quantity of alcohol, called hydrometers, simple gadgets. Further information here:
http://www.homewinemaking.co.uk/hydrometer.html
http://www.homewinemaking.co.uk/hydrometer.html
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