A red blood cell can be produced in four days and lives for 120 days in the blood. Every second several RBCs die and several are released into the blood. If you have a blood transfusion for say a hemorrhage or a surgery, then these RBC will live for 100-120 days. Meanwhile your body will start producing your own RBC. However, if there is some other cause like thalasemmia, hemolytic anemia, or iron deficiency anemia, then after about 90-100 days (earlier in severe cases), you will need further treatment as your hemoglobin level will start falling down. Take care!