How long will the believer struggle with the old nature?

The struggle with the old Adam continues throughout this earthly life. Although God reckoned the old nature crucified with Christ at salvation, in experience it remains and plagues the believer. Only when leaving this body of mortality does the conflict with the old sin nature finally end.

Les teaches that the believer has two natures: the old nature, that old Adam which constantly pursues us, and the new nature received through salvation and union with Christ. This is a Pauline doctrine of constant warfare between the old sin nature and the new divine nature. In Dying Daily, the believer must keep old Adam in subjection and put him to death. Romans chapter 6, verse 13, and the whole sixth chapter of Romans deal with reckoning old Adam as our mortal enemy and putting him constantly to death. God, in His omnipotence and sovereignty, reckoned our old Adam as crucified with Christ the moment we believed; he is dead positionally. But in experience, he is still with us, plaguing us, and he will until we leave this body of mortality.

Why do believers still struggle with the old nature if it is dead?

Believers still struggle because God reckons the old Adam crucified and dead positionally, yet in experience that old nature remains present, pursuing and plaguing them. The new nature from salvation and union with Christ wages constant warfare against the old sin nature until they leave this mortal body.