Did some of God's promises to Abraham take years to be fulfilled?
Yes. God constantly promised throughout Abraham's life, yet some promises took years before they were consummated. Because Abraham still had no son, he grew impatient and succumbed to the temptation to use a slave girl, which led to Ishmael's birth.
Les explains that all the way up through Abraham's life God was constantly promising, repeating "I will." He reminds us that some of these things took years before they were consummated, and that is why Abraham grew impatient and finally succumbed to the temptation to use a slave girl. After all, he could not have a nation of people if he did not have even one son. That delay gave rise to the birth of Ishmael. The same promises to Abraham are also tied to Gentile privilege and future restoration, as Paul says in Romans chapter 11.