I don't know what more experienced owners do, but I use an aquarium thermometer in my dragons' baths to make sure the temp is right. They only cost about $5, and I figure that since they're designed to be accurate in water, it's safer than guessing. I put the dragon in when the water is about 90, then when it gets done to 80 (or they poop) I take them out. Usually about 20-30 minutes.
I have discovered that the more adventurous of the two enjoys getting his back sprayed with a shower attachment. At the end of
bath time, I turn a warm shower spray on and he deliberately swims under it. I make sure it only gets his back and tail, cos I don't want any water in his ears. But it's the cutest thing, and he's absolutely doing it on purpose.