A common guideline is that if you are soaking through a regular pad in one hour for two hours straight, are passing large clots, or become dizzy or pass out, then you need to seek care immediately because these symptoms are not normal.
Bleeding is normal after having a baby, but it should not be to an amount where you are excessively bleeding or becoming symptomatic from it. Reasons that you may be bleeding this much vary. It could be related to an infection in the uterus, or a small piece of placenta could still be in the uterus. Sometimes a laceration or episiotomy can split open, causing bleeding (this can happen on its own or during increased activity or after having sex). Additionally, the site where the placenta attaches in the uterus often involutes around two weeks after delivery, and sometimes this is the cause of brisk bleeding (though this usually resolves as quickly as it comes on).
If you have the above symptoms, during an evaluation your vital signs will be monitored and blood work will be checked. A pelvic exam and/or ultrasound may be done to identify the source of bleeding. Treatments may include medicine to help stop the bleeding, a blood transfusion, antibiotics, or possibly surgery.