Once it begins to reach the low boil, lower the heat to med-low and let it simmer for 12-15 minutes, stirring often to prevent the noodles from sticking. If you notice that the liquid is being absorbed too quickly, add more milk in, about 1/4 cup at a time. Once the pasta is al dente, there should still be a decent amount of liquid remaining in the pot (enough to coat the pasta) and the remaining liquid should be thick and creamy.