When does Paul begin to discuss the Holy Spirit's power in Romans?
Paul does not mention the Holy Spirit's power in the first seven chapters of Romans. That discussion breaks out in chapter 8, where it explodes at the appropriate time. Until then, the Spirit is present but not the focus.
Les explains that in Romans 7 Paul describes the inward man, Christ dwelling within by the Holy Spirit, and the warfare between the new nature and old Adam. Yet Paul has not mentioned the Holy Spirit's power in these first seven chapters. That is going to break out in chapter 8, where it will explode. Les says to be patient and not ask where the Holy Spirit is in Paul's life; He is there, but waiting until the appropriate time. The cry 'Oh wretched man that I am, who shall deliver me from the body of this death?' leads into that chapter 8 answer.