PowerLung is NOT a measurement device. PowerLung IS a training device specifically designed to train the muscles used in breathing.
Training these breathing muscles is no different than training any other muscle. It takes time. Yes, you receive benefits along the way as the strength and endurance of the muscles improve. For the muscles themselves to adapt to the new training and learn the exercise (muscle memory) takes 4 to 6 weeks.
So doing more exercises with PowerLung works the same as overdoing any training. It will not make it better, in fact it might slow your training. It can cause soreness and may even cause injury.