The stages of cooking sugar syrup are:
It takes a long time even boiling at a high temperature to hit 105, and then again to hit 110 C. After that, though, it all happens very, very fast, so you must be very attentive or you will whiz past hard-crack stage even and be approaching carmelization.
It is best to use a candy-thermometer to be sure of exactly where you’re at. However, many people don’t make candy often enough to want to buy or store a candy thermometer, so tests have evolved which involve dropping a small spoonful of hot sugar syrup into a (clear) glass of relatively cold water and guessing at the temperature by how the hot sugar syrup reacts in the water.
High altitude: For every 300 metres (1,000 feet) that you are above sea level, subtract 1 degree C (2 degrees F) from the temperatures given in your candy recipe.