HCG levels are confusing me

So

I was 15 weeks at the time of miscarriage. Exactly 3 weeks after, i had my blood drawn which showed hcg 20. Do you know how long it takes to go from 20 to 0? Also, i know this is a dumb question but i could not find a straight answer, does hcg level have to be 0 in order for af to start? I’m usually very in tune with my body and I feel my period will come by the end of next week. But now I’m confused