As far as therapeutic levels go, you'll know you're getting therapeutic when VS, breath sounds, and overall pt status improve.
When you hear more air exchanging, respirations are less labored, and pt states they're breathing better, you're getting there. When lungs are clear, and there's no SOB, then you're therapeutic. Odds are you might not make it THAT far by the time you get to the hospital. Just get them on the road to recovery is all you can do most of the time.