The correct temperature when steaming your milk is between 140 and 160 degrees Fahrenheit (60 and 70 degrees Celsius). Once you get above the upper limit the milk will be scalded and the flavor of the drink will be ruined. Everyone of course has a personal preference; some like it not so hot, some like it very hot, but be careful with the very hot end; the flavor of your drink will be ruined by going above 160F.
If you are someone who likes to use soy milk in your espresso-based drinks, then the upper limit is 120F degrees (50C).
Most experienced baristas can judge when the milk is at the right temperature by the feeling the sides of the pitcher, but if you want to learn where your "sweet spot" is, get a milk thermometer.
Espresso Coffee Snobs