Google says you should get up to 9 hours of HD video playback, 10 hours of web browsing or e-reading, and up to 300 hours of standby time from the Nexus 7's battery.
In an attempt to quantitatively measure the Nexus 7's battery life in a controlled benchmark environment, we ran a test in which we set up a webpage with a mix of graphics, Flash media and text. The page automatically refreshes every three minutes, we loop the page and also setup a screen lock utility that keeps the display from sleeping during the test. Battery life is measured down to the minute the tablet finally shuts down. The Wi-Fi radio is enabled in this test to simulate battery life in real-world web browsing over a wireless connection.
For this test, we set the Nexus 7's display to 50% brightness, which is still plenty bright and easy on the eyes. The Nexus 7 lasted 7 hours untethered before it powered down.