When you exercise, your blood vessels temporarily dilates up to 400% of it's original diameter specially when you are a youngster. What I can tell you is that avoid fatty foods, your high blood pressure may be caused by fat build up in your blood vessels. Cycling would be a good work out specially planning to lower your blood pressure.
Every human body is different and there's a lot of factors that will come into play. You might have to change diet and the way you live if you want to permanently lower it because you are in a stage what we call "pre-hypertensive"
How long until you reach this goal shouldn't matter. How long you are willing to do this is what really matters.