This depends on the type of soldering station you’re using. If you have a station that has an accurate temperature gauge (whether digital or analog), then the iron will be hot enough when it has reached 660°F. If your soldering station doesn’t tell you the exact temperature, then the iron is hot enough when the solder melts as soon as it is touched. This means that the iron is only hot enough if the solder melts immediately. If it doesn’t, then the iron isn’t hot enough and you need to turn the temperature up.