Running Keynote for presentation give me less then 4 hours on my rMBP 15. Just showing some slides. I have the laptop screen on very low brightness but it does not make any big differences. So I will buy 13 rMBP and hope it will give a full day lecturing without power supply. Perhaps it would be better with new 13 MBA, but if you are used to retina it is hard to settle for less (and of course I do not use retina when doing presentations but there is also other things I want to use the computer for other stuff!) And I also like the horse power and TB2 and so on the new rMBP. Any thoughts!