Hi there!
It's amazing how much hair comes out isn't it? I forgot how long it lasted for me, so I pulled out my copy of "After the Baby's Birth" by Robin Lim. She says that postnatal hair loss usually starts the third or fourth month postpartum, it diminishes gradually, and should stop around the sixth month postpartum. She says that once hormone levels stabilize, the hair should stop falling out, and she recommends eating well and taking a prenatal, as vitamin deficiency can contribute to hair loss as well.
Also, from the Nursing Mother's Companion, with some slightly different info: "Starting at approximately 6-12 weeks after birth, some women experience generalized hair loss. Because of the hormonal changes following birth, hair follicles simultaneously move from the growing phase (which they were in pregnancy) to the resting phase of their development. Postpartum hair loss is seldom severe, and women never go bald because of it. The period of hair loss lasts about three to six months."