This is a difficult question to answer. For starters, you will need an iron with variable heat settings in order to control it. I don’t recommend using a curling/flat iron without a heat setting.
When using hot tools on on fine, fragile or color treated hair, you should select a low heat setting — below 200 degrees — to avoid burning or damaging the hair. Personally, I would recommend a setting under 140°. If you hair falls into the category of thinning or damaged, you should never exceed this temperature. Also, always remember to use a thermal heat lotion as protection from burning the hair.
Bottom line; know the hair you are working with. As you curl your hair, notice how it responds to the heat and increase the temperature accordingly. And never go above 350°- 400°!