Missed Miscarriage - advice please

I'm about 7-8 weeks pg, told last week possible delayed misc as no fetal pole seen on early uss but also have a retroverted uterus which can make us difficult.  Hcg levels have dropped by 600 (still 16300) and started getting a light brown discharge so assuming that I am I fact miscarrying - due to go back Friday for a repeat scan.  Does anyone have any ideas how long it should take for bleeding to start properly or if there is any chance pg could still be viable - still having all the symptoms - I just need to know as seems so unfair