How long after miscarriage to get a negative test

I am 7 weeks pregnant and my Dr is expecting me to miscarry again. The first time I miscarried I bled heavy for 5 days. I bled heavier than spotting but not even heavy enough for a tampon on Friday with some small clots. I tested this am and it's still positive. I have an ultrasound on Monday but this has been a 2 week long process and I'm impatient. Just wondering what everyone's experience was. Thanks!