How to Set the Automatic Lock Time on Your Nexus 7
You can manually lock the Nexus 7 at any time by pressing the Power Lock button. When you don’t manually lock, the tablet automatically locks itself after a given period of inactivity. How long? That’s up to you, though the Nexus 7 is preconfigured to snooze (self-lock) after ten minutes of your ignoring it.
You may find that it is more handy to have a different time-out setting. To set the time-out to something other than ten minutes, obey these steps:
At the Home screen, touch the All Apps button.
Choose Settings to open the Settings app.
Choose Display and then Sleep.
Choose a time-out value from the Sleep menu.
The range is from 15 seconds to 30 minutes at various intervals.
Touch the Home button to return to the Home screen.
The lock timer begins after a period of inactivity, when you don’t touch the screen or the timer starts ticking. About five seconds before the time-out value you set (refer to Step 4), the touchscreen dims. Then it turns off and the screen locks. If you touch the screen before then, the timer is reset.