OK, here is a link to a couple of run time graphs (high and medium mode).
http://www.candlepowerforums.com/vb/showthread.php?380017-Fenix-E12-run-time-graphs-compared-with-other-popular-lights
This was a thread comparing the Fenix E12, but since the PA10 was one of the most efficient lights I have ever tested, I included it. You can see that it was 3rd most efficient. The listing of lights in the graph is arranged from most efficient to least efficient. I will say, I've never owned a Zebralight but I think they are right near the top if not the most efficient.
As you can see the light is current regulated and holds a very steady output. On high the standard AA battery such as an alkaline will start out as regulated but will drop out sooner than the Eneloop battery. On medium the 2 will be closer to each other.