I get about 2.5-3 hrs of continuous screen time out of a full charge on my GNex. That is "Heavy" use. When that number reaches 8-10 hrs, I will consider my battery adequate. I believe that is going to take new screen technology, more efficient processors, and better battery tech as well.
But that is my standard. At least 8 hrs of "screen on" time from one charge. I'm a third of the way there.
Sent from the red headed stepchild of the Nexii.