Adding Noodles to Soup
Whether you choose to cook the noodles in the soup, or separately, once they are in the soup, you want to serve it immediately.  Either way, they will continue to absorb water and begin to grow mushy.  (this is why it is better to refrigerate or freeze soup without the noodles, and add them a la minute when you are heating it to serve later).

Because of this, if you do choose to cook the noodles directly in the broth, you would do so about the same amount of time before it is done, and then serve the soup immediately.

As long as the soup is above about 180 F / 82 C the noodles will be cooking at close to full speed, so if you are going to have hold time before service, you should count this in the cooking time.