it is impossible to say with certitude, but here's the deal
. three to five days seems to be the answer.
as you can see from the link... it seems that heavy users are more likely to test positive at, say, three days, than the occasional user.
i can see that you are very nervous about this and i hope you skate by. i'm not going to preach at you about drug use except to say that of all the drugs of abuse, cocaine, can be that lightning bolt that kills you with just one line. heroin
can certainly do it but users of heroin die from respiratory depression due to overdose... cocaine can just flat out screw your heart up and kill you with no warning.
now, let's say you test positive, what do you do? first of all, you might request a retest. you might mention the amoxicillin thing as i do not think it is completely disproven by the nih study. and, if you get to retest do not, do not, do not use any cocaine or meth or anything like it. drink a lot of water the morning of your test but fully empty your bladder before going to the test.
other than that i hope it all works out for you. you were at the three day window when you gave your sample. if you are an occasional user then there's a decent chance you will test negative.