Everyone is so different that question would be hard to answer. Severity of the disease, Length of time on steroids, your exercise, stress level, diet, other auto immune conditions, your overall health. All are contributing factors. Prednisone can have life long side effects. Your Dr is good here to help figure out. The idea just as you have attempted is to find the lowest possible dose. I would say with some discomfort just to be at your lowest. You should continue to try to move that bar. You should probably go to 20mg for a couple days. The prednisone acts very quickly. Then try to get back to 15mg or 12mg fairly quickly. After 2 weeks your body has shut down production of cortisol since the prednisone is supplying it. Get comfortable at 15mg before going to 12 this time. Slow the taper. PMR can take some time to burn out. Mine took 6 months. But many have trouble for years.