Reduce the heat and simmer it without stirring. This is important: do NOT stir the mixture! After 15 minutes (or over an hour, in my case), test the temperature with a thermometer
. Once it reaches 235°F or soft ball stage, remove from heat. Do NOT stop simmering until it reaches this temperature, or it will not set. It will be noticeably thicker at this point, but still slightly runny.