How many weeks are required approximately for complete uterine involution during puerperium?

Uterine involution is nearly complete by 6 weeks, at which time the organ weighs less than 100 g. Myometrial contractions, or afterpains, assist in involution. These contractions occur during the first 2-3 days of the puerperium and produce more discomfo in multiparas than in primiparas. Such pains are accentuated during nursing as a result of oxytocin release from the posterior pituitary. During the first 12 hours postpaum, uterine contractions are regular, strong, and coordinated. In case of abnormal uterine involution, infection and retained products of conception should be ruled out. Ref: Pessel C., Tsai M.C. (2013). Chapter 10. The Normal Puerperium. In A.H. DeCherney, L. Nathan, N. Laufer, A.S. Roman (Eds), CURRENT Diagnosis & Treatment: Obstetrics & Gynecology, 11e.