just below a simmer, i do mine for 30-40 minutes, but i have a gas range and can dial in the perfect temp almost to the degree. if you see any smoke at all turn down the heat a bit. the idea is to have just as much oil at the end as when you started. the oil should be brown, not green or clear. if you can see through your oil you wasted your time. if you end up with less than you started you cooked at too high a temp and burned some off.
edit: i tried to include a pic of what the oil should look like but either the site is having an issue or my isp blows.