In most "ordinary" disasters like earthquakes and hurricanes, local pharmacies begin to re-open within days, and tend to receive priority assistance in doing so because of their importance to their communities. Within a few weeks, there are usually a large fraction of the normal number operating in or on the periphery of the affected area. So a three month supply is usually going to prove more than sufficient.

Of course, in the event of a large meteor strike, alien invasion, thermonuclear war, or the like, it may take slightly longer for your local pharmacy to get fully back to normal.