The cornstarch will activate and thicken the curd within a few minutes of cooking on the hob. If you're using a gas hob, it may thicken a little quicker. To tell whether the curd is cooked, it should coat the back of a rubber spatula. You should be able to draw a line down the curd with a spoon and it should hold its shape. At this stage, remove it from the heat and whisk in the vegan butter. The solid fat in the butter will also thicken the curd as it cools. The curd will set further in the fridge to a pudding consistency, which takes an hour.