We've conducted some tests to help people get a feel for how long AA batteries should power a Rasberry Pi in a variety of configurations when using MoPi. We don't have results from the B+ yet, but that's coming soon...
For now though, the best case scenario? 50 hours using the Model A with 16 AAs!
That's best case, of course, with the Pi not doing anything much, but it does illustrate the potential.
Read more in the main article!