How long does metoprolol 100mg take to lower blood pressure?

Try 2 weeks: May be needed another agent as well.

Short and Long Term: Well, the drug takes effect in ~2 hrs, but there is a development of effect over time when starting it, with peak effects not occurring for a few weeks. That said, beta blockers (and metoprolol in particular, second maybe to atenolol) are poor choices for first line treatment of blood pressure. A diuretic like hydrochlorothiazide or an "ace inhibitor" is usually a better first choice. Ymmv.