How long does it take to naturally bleed out after a missed miscarriage

In my first ultrasound at 7 weeks, there was no heartbeat found, however since the yolk sac was very strong the doc wanted to give the benefit of doubt.. 2 weeks later in the 2nd ultrasound the fetus had grown slightly but again no heartbeat.. due to this growth in the 2 weeks i have been called in for a 3rd ultrasound and the doc refuses to call it a missed miscarriage.. im fed up of these disappointing hospital visits.. how long does it take to naturally bleed out in a missed miscarriage??#adviceplease