How long for HCG levels back to 0?

This week I had a D&C done. At 6 weeks (and then confirmed again at 7 weeks) our little baby had no heartbeat. Dr says we can try again for our BFP once my HCG levels go back to 0.  I was wondering, for all those others out there who suffered a miscarriage, how long it took for those levels to hit 0.