was to stop the estradiol shots at 10 weeks and at that point drop back to every other day on the PIO until 12 weeks. When we saw the OB when we had about 2 shots of the PIO to go, she said just to quit then. (I don't know who was more relieved, me or DH, who'd been giving them to me!)
If you're also on suppositories, that will make a difference. (I was not.) Did your RE tell you to continue those after ending the shots?
As I understand it, the placenta begins making progesterone around 8 weeks. I was still nervous with tapering off at 10, but all has been well.
One other thing: the RE never mentioned this, but the nurse practitioner at my OB's office mentioned that you can have a "withdrawal bleed" after coming off the progesterone. That would have been scary, esp. since I'd had some spotting already, but thankfully I did not have it. I've been off the PIO for nearly 3 weeks now.