How long did Israel have the mindset that they were God's special covenant people?
Israel held that mindset from the initial promise to Abraham until their dispersion after rejecting their king. Throughout that entire period, it was constantly reinforced that they were a special, called-out, separated covenant people of God, with all their promises being earthly.
Les teaches that as soon as Abraham was called out of Ur of the Chaldees, God separated him from the mainstream of humanity and promised that out of that one man would come a distinctive race and nation, Israel. From the very onset of the promise to Abraham until they were finally dispersed after they had rejected their king, Israel had it constantly drummed into their mindset that they were a special, called out, separated, covenant people of God, and all their promises are earthly. Therefore, they are referred to as God's earthly people. Les clarifies that Old Testament saints went to paradise, not heaven, and that even in their resurrected state, Jews will be an earthly people, not a heavenly people, so all of Israel's promises were earthly.