How long is too long to wait for a missed miscarriage?

I was told 7 weeks ago that I have a missed miscarriage. I found out at 8 weeks and measured at only 6 weeks. I haven't had any cramping or spotting. I've had 4 ultrasounds and 3 blood tests over the last 7 weeks so it's definitely a miscarriage but I'm getting worried it's been so long. My doctor didn't seem worried about infection since it was a blighted ovum.