I am 6 weeks my hcg is 200 no gestastional sac and i have been told to expect to miscarry. how long will that take to happen?

Be patient/cautious: As long as the pregnancy is/was in the uterus, you should eventually pass the tissue. It can take a few weeks. It is always a concern with a low beta-hcg that there is an ectopic pregnancy. You wouldn't expect to see anything with the ultrasound now. Get follow-up beta-hcgs every 3-5 days to see if they are risisng, falling, or staying the same.