For large baking potatos it can take 4 hours, the smaller the less time its going to take. specially if they are cooking in a broth liquid which adds more flavor to the potato.
So say you have a 6 lb roast that you are going to cook for 8 hrs and after about 4 hours of cooking you add your potatos, carrots and celery, when its all done your potatos will be fork tender and delishous. Or atlest thats what happens to mine hehe. References :