I didn't test after my first D&C, but I ovulated 5 weeks after. Since a body won't gear up to O until there's no more hcg left, then I'm guessing it took 2-3 weeks for my levels to go to 0.
After my second D&C, I did test 17 days after (2.5 weeks) and still got a faint positive.

I guessed that by the end of the third week it would be negative and I think I ovulated a few weeks ago which would make that about right...I've heard it hits about 0 when you finally stop bleeding and spotting completely. I don't know how long is normal!
Good luck to you. hugs.