There are many articles and Reddit posts saying to heat the cast iron on low for a long time and then cook in it. Apparently it’s a bad idea to heat the cast iron on high as heat doesn't flow easily through the metal bulk. So, how long should I heat up the cast iron? Normally I put a few drops of water in the pan to check the temperature, but clearly that is not a good idea with cast iron. 
You heat it until it has reached the heat you need for the recipe you are going to make. It doesn't matter how long. You can use any method you like to decide when it's ready. You mention that you are comfortable with a water drop method; that's great. You can use it on your cast iron pan, there is nothing to speak against it, and it will work. Also,  Apparently its a bad idea to heat the cast iron on high  This is nonsense. Of all pan materials that exist, cast iron is the one that is most forgiving to overheating. It also takes longer to preheat than others, so using a high setting for preheating is really practical. 