It's hard to say as it depends on how quickly your body can shift the RPOC and to some degree how the miscarriage happened. I had RPOC for about 5 months after an MMC (blighted ovum/empty sac found at 5 weeks) and I was told to keep a close eye out for any symptoms of uterine infection.
I didn't have surgical intervention as I wanted to see if my body would clear it all out on it's own. It did, but the random haemorrhaging was no fun, and seeing stories of people having had surgical intervention and being left with tissue remaining, it just seemed better to wait and deal with the bleeding. I had scans every few weeks to track progress, from which I could see highly-vascular tissue (bits of the placenta with lots of blood vessels that were slowly degrading) getting smaller and smaller.
After that was all over, my period came back and it was about a year before I conceived again, which sadly ended in another MMC, but that one resolved very quickly as there was no empty sac, but sadly the baby failed to develop beyond 6 weeks. Once I'd come off the progesterone, my body flushed everything out. A year later, I got pregnant again and I'm 28+4 today.
TL;DR :
The time it takes to resolve RPOC depends on how the miscarriage happened and how your body deals with RPOC.
Final comments:
The three worst aspects of an MMC for me personally are the pain, the bleeding and the emotional side of it all. Every time your body gets rid of a bit more tissue, you're reminded once again that you lost a baby (another baby in my case) and it's heartbreaking.