how long does it take to naturally miscarry?

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Hello everyone, I am having my second miscarriage and with my first everything went so fast in the ER that I wasn't able to miscarry naturally. 
This time after bleeding for 2 weeks and 2 ultrasounds, they told me that my baby no longer had a heartbeat and I told them I wanted to pass it naturally this time. 
It has been 17 days since the bleeding started and 5 days since they confirmed the miscarriage. 
I'm still only spotting and I'm hardly cramping at all. 
How long did your natural miscarriages take to pass? 
After how long with no progress should I go back to the doctor?