You should try to grow rock candy in different temperatures to find this answer. Make up one large batch of the supersaturated solution of sugar and water. Separate it into three or four containers, place each in a different temperature environment (a cold garage, a cool basement, a warmer room, and maybe the kitchen near the stove (usually fairly warm there) or in a sunny window. Use a thermometer in water to get an accurate reading of the temperature in each area (2-3 hours per area will generally work fine, just be sure to take the reading at the beginning, midpoint, and end for best results). Place in the containers the string or stick you will use to create the rock candy and wait for 5-7 days. Do at least 2-3 strings/sticks in each container for best results. At the end, compare the amount of rock candy deposited on the string/stick per container and then decide which temperature grew the most. This will take about a week, so I hope you didn't wait too long!