Do you have to wait twelve hours before you can take another propranolol?

If it is prescribed for you that way, then yes. However, there a number of individuals who take very low dose propranolol, not for reduction of blood pressure, but for anxiety/phobia involving public speaking. In this instance, very low dose propranolol may be taken several times per day, as needed. (N.B. These doses are on the order ot just 10-20 mg, rather than the significantly higher doses utilized for the treatment of hypertension).