Fudge should reach 234° (soft ball stage) on a candy thermometer and then be removed from the heat. The soft ball stage ranges from 234° to 240° . You don’t want to overcook the fudge. If you don’t have a candy thermometer, you can try an old version of testing by dropping a small amount of fudge that has boiled for about 10 minutes into a glass of cold water. It’s reached the soft ball stage if it forms a ball and does not form any “threads.” It should also squish easily. (Nigella Lawson’s website has more info about fudge temperartures.)