I found out at 8 weeks that I had a blighted ovum and didnt start the miscarriage process for about 4 weeks after finding out. The whole process of the miscarriage took about 2 weeks. Miscarriage is a very hard thing to go through, I had the option to do the D &C but opted out of it my mind kept thinking that maybe something could change. Through out the whole process I kept wondering if I should just go ahead and do the D & C but my body finally finished it on it's own. It is different for every person, It's up to your body on when how long it decides to take.
It took me 3 weeks, but I didn't know I had a blighted ovum until I started to miscarry.
Every women is different. Some take a few days or a week, some take 4 weeks, some never do and need a d&c. The process itself could take a few days up to a few weeks as well. (The actual bleeding, not the waiting to bleed part)