Bleeding with previa for weeks off an on. When and how do obs decide to deliver?

Varies. The timing of delivery will depend primarily on how much the patient is bleeding, with consideration given to how far along the pregnancy is. When very preterm the bleeding may be managed with transfusions - if the blood loss exceeds the ability to replace the blood, delivery is required. With less bleeding, observation may be continued until after 34 to 36 weeks.
Individualized mgmt. Depending on the amount of blood loss, your maternal-fetal medicine specialist will decide when to pull the plug on the pregnancy and proceed with a cesarean section. We have to take the fetal interests in mind as well, as we try to avoid severe prematurity if possible. Typically, placenta previa is managed by prelabor cesarean section by 36-37 weeks at a large hospital.