I was on them til 8 weeks with dd2 and 10 weeks with this pregnancy. Once the placenta takes over at 8 weeks you are generally fine. Tell your health care pro if you are worried, I spoke to mine who was willing to let me keep going but I felt it was fine.
My friend was on them to 30ish weeks as she had had a 30 week baby previously.
Everyone and every pregnancy is different .