I aim to let this soup simmer for at least 45 minutes so the flavors can meld, but if I have the time I let it go for an hour or a bit longer. Just keep the pot covered (or keep the lid open just a crack) to prevent the soup's liquid from evaporating. No matter how long you let the soup simmer for, just be sure to add the orzo during the last 10 minutes so it doesn't overcook and become mushy.