My Galaxy S5 Active, in Ultra Power Save mode would last 12 days according to it's in phone estimate. I was never able to push it in that mode the whole way through, but 2 days sipped about 10 percent of my battery when I tried, so I fully believe it was possible on that device. It downclocked the processor, turned basically everything off, like you said, and even lowered the display resolution and color to greyscale. I don't think I've seen a later phone than that with such a huge Ultra Power Saver.