The lemon curd is done when it has thickened to the point where it can coat the back of a spoon. When you dip a spoon into the curd and lift it out, the curd should cling to the spoon instead of immediately dripping off. Sugar free lemon curd will continue to thicken as it cools, so it doesn't need to be extremely thick when it comes off the stove. Just remember to stir it continuously over medium heat to prevent it from burning or turning into scrambled eggs.