I also have UX305 and I easily get at least 6 hours under moderate loads. Eight or nine with light web-browsing (avoiding javascript pages that spike cpu usage), ten or more with lighter use (text editor).
I installed Ubuntu 14.x and i3 window manager. I have dual boot with original Windows install and power use is comparable between Windows and Linux. It's a pleasure to use. Only power management tweak I made in Ubuntu was to install the TLP power management tool and (I think) make simple change from its defaults: http://linrunner.de/en/tlp/tlp.html