Chemical

How long did it take for your line to completely be negative after a chemical? I had a faint line 4 days before my period was due, bled for one day the day before period was due and then felt shitty so took a test on the 18th which I’ll post- it was positive...now it’s the 21st and this afternoon I thought I would take a FRER and it was completely negative...more than likely a chemical?? So my tests were positive for two weeks and I had some small line progression from tues pm and weds am but negative this morning I’m confused......