If you know the exact amount of sugar and water, atmospheric humidity, and heat supplied you can create time tables of how long to boil. Since it is nearly impossible to control the atmospheric humidity and oven top burner heat transfer it's nearly impossible to come up with TIME based boiling criteria. The most popular method is the Water Test which tests the mechanical characteristics of the sugar syrup. A few drops of hot sugar syrup are dropped into cool water and allowed to cool. Fish out the cool syrup and roll it between your fingers and determine it's character.
Temperature measurement is probably the most accurate and reproducible across the nation.
Novice candy makers should use a candy thermometer until they develop enough experience working with the sugar syrup. We use the candy thermometer for making fudge even though we use the Soft-Ball test to determine when to stop boiling. Using the thermometer acts as a confirmation. Sugar syrup is the foundation for most candies, the concentration of the mixture depending upon its temperature, which can either be checked by a candy thermometer or by a series of cold-water tests . To sugar-coat various fruits, flowers and plants such as cherries, pineapple, citrus rinds, angelica, ginger, chestnuts, violets, miniature rose petals and mint leaves.