With your history of two miscarriages, I will usually be very conservative and continue the progesterone until 12 weeks gestational age. However, medically, it would be okay to stop at 10 weeks. By then, the placenta should be fully functional and providing all the hormone necessary to maintain the pregnancy.
Crinone vaginal gel is also used to support pregnancy in women having in vitro fertilization (IVF). In this case the gel is used daily, starting after the embryo has been transferred into the womb, for the first 30 days of confirmed pregnancy." www.netdoctor.co.uk/pregnancy/medicines/crinone.html