I have not seen article(s) about clearance time of amniotic fluids. It seems that except for the water which may be exchangeable with mother, all other things stay there until the end of pregnancy.  Amniotic contents are produced by the epithelial cells of the upper and lower respiratory tract and mouth of the fetus, some urine, and in the female fetus, some from vagina of the fetus. We should add products and death cells of the amniotic cavity as well. The contents may be swallowed by the fetus and digested in the alimentary tract of the fetus thereby, producing meconium. You may find  meconium, hyppomans and amniotic plaques in the amniotic fluids and they are never absorbed until parturition.