How long for the uterus to shrink back to pre-pregnancy size if you are breastfeeding?

Usually by 4 weeks: The uterus does naturally return to normal size by 6 weeks post-partum, but breast-feeding releases a chemical in the brain called oxytocin, which helps to contract the milk ducts in the breast to assist in nursing. This oxytocin, as a side-effect, also can cause "after-birth pains" which helps the uterus to contract and shrink back to normal size.

Generally 6 weeks: In general this takes about 6 weeks if you are nursing or not, it probably goes a little faster if you are nursing since you release more oxytocin to make the uterus contract.