my MW said I could start orally and vaginally taking them at 35 weeks.... I think she said 2 orally and 1 vaginally, though she said wear a pad and poke a hole in it with a safety pin... I haven't done it this time around though just because I'm being lazy and they are packed in a box because we are trying to move....
ETA: I used them last pregnancy and didn't go into labor until I had my membranes stripped at 41w5d... it might have made me favorable but I don't know if it really was the EPO... can't hurt though