I have done some tests, not on a 15 but a fully specced retina 13 (mid 2014 model).

If I am running a couple of iTerm windows, chrome for the dev tools (even though this eats up more battery then safari) with a couple of tabs open and sublime tex i'll get about 6 hours on average running my normal workflow. If I switch to Atom instead of Sublime I get just about 5 hours.

This is down to ~10% before anxiety kicks in and I have to return to the cord.