It shouldn't be doing this! :-) My tablets both stay on for days at a time if I don't use them, I've never had them switch themselves off. (Incidentally, I assume you never switch your tablets off?) If you want to isolate the problem you might want to try reinstalling the firmware. After the reinstallation, don't alter any of the settings and don't install any apps. See how long the tablet lasts when it's still "plain vanilla".