Abram believed God, then God counted that to him as truth (Genesis 15:6)
Every mother can say that waiting for birth is a patience-building experience in her. How unfortunate for the mother elephant, because it takes about 22 months to conceive a fetus until it is ready to be born! A type of shark known as the finned dogfish has a gestation duration of 22-24 months. And, at altitudes above 1,380 meters, the Alpine salamander can have a gestation period of up to 38 months!
Abraham's feelings may not have been much different from examples found in nature. At his advanced age, God promised him, "I will make you into a great nation" (Genesis 12:2). However, as the years passed, Abraham questioned how this promise could have been fulfilled without a son (15:2). So then God assured Abraham that, "Your biological son ... will be your heir" (verse 4).
Even though he was getting older, Abraham still believed in God, so he was called righteous (verse 6). Nevertheless, he waited for 25 years since the first promise about the birth of Isaac (17:1,17).
Waiting for the fulfillment of God's promises is part of trusting Him. No matter how long the delay is, we must still wait for Him. It is also warned by the writer of the book of Hebrews to us, "Let us hold fast to the confession of our hope, for He who promised it is faithful" (Hebrews 10:23) --MW
GOD ALWAYS FULFILLS HIS PROMISES
*) Take from Daily Reflections