How long did Abraham and Sarah wait for Isaac to be born?
God had been promising Isaac for 20 some years, and Abraham and Sarah had to patiently wait. Even when they were past the time for childbearing, God miraculously gave them Isaac. So they waited roughly two decades, possibly a bit longer.
Les explains that in Galatians 4, Paul uses Abraham’s two sons as an allegory: Hagar the bondmaid bore Ishmael after the flesh, while Sarah the free woman bore Isaac by promise. Les points out that God had been promising Isaac for 20 some years, so Abraham and Sarah had to patiently wait. Even though both were past the time of childbearing, God miraculously gave them Isaac. This contrast highlights law versus grace, flesh versus promise.