When does the battle between the old and new nature begin and end?
The battle between the old and new nature begins the moment a person is saved. It continues until the believer leaves this life, either by death or by the Rapture. Until one of those occurs, the two natures remain contrary and in constant warfare.
Les teaches that the old nature declares war on the new nature, so the new nature has to declare war on the old nature, making it a constant battle. The battle lines are defined from the very moment of salvation until we leave this life, either by death or by the Rapture. Until then, believers are in constant warfare. The old Adam, or flesh, lusts or wars against the Spirit, and the Holy Spirit wars against the flesh, or old Adam. These two forces within every believer are contrary and at total odds; they cannot agree on anything. The new nature has a whole new set of values in compliance with the righteousness, holiness, and laws of God, while the old nature, whose god is Satan and the god of this world, is bent in rebellion and unbelief. They are two total opposites and war constantly.