Hello, 2 months ago I reduced my Benicar (olmesartan) from 40 mg daily down to 20 mg daily. Is 1/2 of the drug out of my system by now?

Medications. I do not understand the way you are seeing things. The drug is still in your system. There is a therapeutic dose range that it get's to. If this dose is therapeutic then your blood pressure should be normal. On the dose you are currently on that would have occurred within one week of the change.