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)
It took me 3 weeks, but I didn't know I had a blighted ovum until I started to miscarry.
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.