You're welcome to use a candy thermometer or high quality instant read thermometer if you like. If so, I recommend cooking your caramel until it's just about to reach 350°F (177°C). I find that by the time I manage to take the pot off the heat and pour in the cream, the caramel has cooked slightly further and is exactly as dark as I like it. Some experts recommend taking the caramel between 350° and 355° before removing it from the heat. 350°F is the point at which sugar starts to burn — you'll see smoke start to waft from the surface — so if you're taking it this far, you'll need to work quickly so it doesn't tip over into extreme bitterness. In my experience, waiting this long results in a slightly more bitter taste than I prefer, but your mileage may vary. Most of the time I don't use a thermometer at all. I find it simpler to use visual cues. Color changes happen quickly at the end of caramelization, so I like to just focus on watching the color and removing the pot from the heat just as it reaches my preferred shade of dark amber.