Let it come to a simmer. Once the sugar is dissolved and it’s come to a boil, bring the heat down to low and let it simmer for 8-10 minutes or until the sauce begins to thicken. A good way to tell if the sauce is done is by coating the back of a spoon. If it stays coated, it’s done. Remove from the heat immediately. It will continue to thicken as it cools. Serve chilled over your favorite sushi.