Really depends on your phone and how much you use it. If you don't navigate with it, and keep it in airplane mode while you're riding (which I sometimes do to prevent it from running down while searching for a signal in the Middle of Nowhere) then I find I can get pretty good life out of phone. I think 8 days would be optimistic, though. Half that if I'm lucky.

I've been in situations where I had to go 3 days or so without an outlet, but it's rare. If I were to go without finding an outlet for days on end, I'd probably also worry that I wouldn't find my AAs. I don't want to carry a weeks worth of AAs even if it would work to keep my phone charged. So I charge my phone and/or my extra battery whenever possible.

I also have a set-up where I can charge my external battery off of my dynamo hub, but I really haven't put that to the test. I've also considered a solar charger for days when I'm stationary, because that's the real problem. Sitting still means not coming across an outlet anytime soon and also means more time to fiddle with my phone, so it runs down faster.

Basically I think plugging in to outlets you find along the way can be a solution, but it's not always an ideal solution. But I still think it's better than endlessly chewing through AAs.