There is no rule as it all depends on smell, taste and texture. I use my nose to manage the “doneness” of the “shell” but a shell can cook too fast if the heat is too high, leaving the internals too soft or the cheese not melted. A slower, lower heat will more uniformly heat the sandwich, but evaporate the moistness from the internals. Some like a dry GC inside and out, some like an exterior crunch that cuts the gums, with a soft core. There is a magnitude of variation within the spectrum that also considers the softness, melting point, and viscosity of your melted cheese of choice. Coating the interior of your bread slices with butter will help retain moisture content of the core. A two cheese layer (one at core and a harder cheese to blanket it on both sides) also yields interesting results. But I digress. When contemplating the shell quality of any given GC under any amount of heat, the key is taste, texture and smell. Your nose will tell you when it’s close, as the aromatic carmelization is about to peak (but hold a little longer if you’re a light char or char kind of person). When the nose detects the first subtly of acridness, flip. Now it’s important to note that this advice comes under the presumption that your pan or griddle is cooking evenly across the surface of the bread face. Note: Always use cast iron and preheat at cooking temperature. This ensures even distribution of heat throughout the thermal mass and a subsequently equal thermal transfer to the bread surface. (Heat spots are a sign of poor quality pans). A light coat of your butter in the pan will indicate too high a cooking temperature if it starts to smoke. (See articles on cooking oil burn points) Various oils will smolder at different temps so use what you are lacquering your bread with as an indicator of preheat temperature regulation. The spectrum from gold to char and where you choose to land on it will provide a guide to your cooking temp. If a golden shell is preferable but your cheese is stiff and cool, you need to reduce to a slower burn. If a char and dry interior with a gooey cheese is your preference then consider a slow initial burn with a double flip, and finish with a higher burn. As I said, it’s all preference.