How long does a chemical last?

I got a positive test on the 21st February. The tests were very faint and never really got darker so I got my hcg checked last Thursday and at around 5 weeks, so maybe 14dpo my hcg levels were only 64. I was told to make an appointment in a week to recheck, so I have a blood test this Friday.

Unfortunately I started bleeding last Friday, a day after the test so the pregnancy has clearly ended. But I took another test just to be sure and the tests are still pretty dark, and some even more than at the beginning. I thought once I started bleeding it would just be negative now. Anyone know how long it'd take to go back to 0 from 64 if I've already started bleeding?