How long was Israel under the Law and what was God doing during that time?
Israel was under the Law for 1,500 years. During that time God was training and teaching the nation, aiming to bring them to maturity as understanding, believing Jews. The Law's purpose was to prepare Israel to be vessels and instruments God could use to reach the Gentile world.
Les explains that for 1,500 years Israel was under the Law, and it was a system by which God was training them, teaching them, and hopefully bringing them to the place where they would be mature, understanding, believing Jews. Les notes that this did not happen, but it was what God was striving for. The purpose of the Law was to prepare the nation of Israel for a great opportunity—to be vessels and instruments God could use to bring in the masses of the Gentile world. He also clarifies that while “the Law” can sometimes refer to the whole system of Judaism—Ceremonial, Civil, and Moral—here Paul is making reference only to the Moral Law, the Ten Commandments, as the schoolmaster to bring us to Christ.