It took me about 8 weeks but I had a small amount of retained products
MMC @ 10w, had an ERPC the same day
4 weeks later I'm still getting positive tests, my Dr scans me and can't see any evidence of any retained tissue, and I get bloods done
hCG levels at 4 weeks post ERPC were higher than when I was 4 weeks pregnant!
We agreed a two week cut off: we'd monitor my levels, and if hCG wasn't down to zero within a fortnight then we would do a hysteroscopy to take a closer look.
My levels did drop, but slowly, so @ 6w post ERPC we did a hysteroscopy. Sure enough there was small fragment of retained pregnancy tissue (so small it wasn't showing up on ultrasound). 2 weeks post hysteroscopy (ie 8 weeks post ERPC) I finally got BFN.
We went straight into another round of IVF so couldn't tell you about getting my period (and PCOS means not having cycles anyway)
Wishing you lots of luck and very sorry for your loss 